Open files and subagents in a closable sheet on phones (#6827)

* feat(app): open files and subagents in a closable sheet on compact

On phones, opening a file, a provider subagent, or a Paseo subagent now shows it
in a bottom sheet over the current tab instead of adding a tab, so the tab strip
stays for terminals and root agents. Paseo subagents get "Open as tab" to move
the same tab (same tabId and state) into a full tab with a composer; agents in a
sheet hide the composer.

The decision lives in the workspace layout store's openTab, driven by a
per-kind `compactSheet` rule in the panel manifest, so every open path (chat
links, Explorer, subagents track, notifications, plugins) follows it without
call-site branching, and a new kind opts in with one manifest entry. The app
shell publishes the layout mode (compact / single-pane / split-panes) once, so
callers stop passing isCompact for placement.

A sheet holding unsaved edits is never implicitly displaced; later opens go to
tabs underneath, and closing it goes through the existing discard guard. The
editor host is position: sticky because CodeMirror's scrollIntoView otherwise
scrolls the bottom sheet's container and drags the sheet to full height.

* test(app): fail the compact sheet snap check when the sheet is oversized

---------

Co-authored-by: paseo-bot[bot] <paseo-bot[bot]@users.noreply.github.com>

CLAUDE.md

@@ -54,7 +54,7 @@ At the start of non-trivial work, list `docs/` and skim anything relevant to the
 | [docs/qa.md](docs/qa.md)                                             | QA evidence bar for pull requests — platform matrix, version drift, performance, UI proof                                      |
 | [docs/mobile-testing.md](docs/mobile-testing.md)                     | Maestro and mobile test workflows                                                                                              |
 | [docs/mobile-panels.md](docs/mobile-panels.md)                       | Compact left/center/right panel ownership, worklet motion, gesture revisions, and Fabric constraints                           |
-| [docs/explorer-sidebar.md](docs/explorer-sidebar.md)                 | Explorer sidebar and ordinary side-pane host contracts, lifecycle, placement, and routing preferences                          |
+| [docs/explorer-sidebar.md](docs/explorer-sidebar.md)                 | Explorer sidebar, side-pane, and compact workspace sheet host contracts, lifecycle, placement, routing preferences             |
 | [docs/ad-hoc-daemon-testing.md](docs/ad-hoc-daemon-testing.md)       | Isolated in-process daemon test harness                                                                                        |
 | [docs/browser-capture-harness.md](docs/browser-capture-harness.md)   | Real-Electron browser screenshot harness and compositor-surface gotcha                                                         |
 | [docs/android.md](docs/android.md)                                   | App variants, local/cloud builds, EAS workflows, version codes, F-Droid source builds and store metadata                       |

docs/agent-lifecycle.md

@@ -212,7 +212,7 @@ parentAgentId === thisAgent.id  AND  !archivedAt
 
 - **Provider subagents** are child executions owned by Claude, Codex, or OpenCode. They are not inserted into `AgentManager` as managed agents. Providers emit a separate descriptor and timeline stream through `agent.provider_subagents.*`; the client keeps that state outside the normal agent store and merges only the presentation rows into the track. A descriptor's optional `parentSubagentId` identifies its direct provider-subagent parent; an absent value identifies a direct child of the managed agent.
 
-Clicking either kind opens a workspace tab. A Paseo subagent tab is a normal interactive agent pane. A provider subagent tab is a read-only timeline pane with no composer, archive, detach, rewind, or fork actions. It shows its own direct children in a subagents track. Both panes use `AgentStreamView`, so message, reasoning, tool-call, and layout rendering stay identical.
+Clicking either kind opens a workspace tab. On compact layouts it opens in the [workspace sheet](explorer-sidebar.md#workspace-sheet) instead; the sheet hides the composer, and a Paseo subagent's sheet offers **Open as tab** to get it back. A Paseo subagent tab is a normal interactive agent pane. A provider subagent tab is a read-only timeline pane with no composer, archive, detach, rewind, or fork actions. It shows its own direct children in a subagents track. Both panes use `AgentStreamView`, so message, reasoning, tool-call, and layout rendering stay identical.
 
 Provider timelines use the same structural timeline item format but deliberately have a separate lifecycle and transport. A provider thread/session identifier is not a Paseo agent identifier, and closing its tab is always layout-only.
 

docs/explorer-sidebar.md

@@ -7,6 +7,7 @@ contracts.
 | ---------------- | ---------------------------- | ------------------------------------------ |
 | Explorer sidebar | Files and Changes navigation | Cmd+E shows or hides the dedicated dock    |
 | Side pane        | Ordinary workspace content   | Created and closed like any workspace pane |
+| Workspace sheet  | Compact look-ups             | Holds one tab; closes or becomes a tab     |
 
 ## Panel host contract
 
@@ -22,6 +23,24 @@ can move between Explorer and main panes.
 Keep panel implementations independent of either shell. `WorkspacePanelHost` owns mounting and
 retention, while each shell owns its tabs, focus, dragging, resizing, and shortcuts.
 
+## Workspace sheet
+
+On compact layouts, files and subagents open in a closable sheet over the workspace, so the tab
+switcher keeps only agents and terminals. A panel kind opts in with `compactSheet` in its manifest
+(`packages/app/src/panels/panel-manifest.ts`), and the layout store's `openTab` decides between
+sheet and tab, so callers open targets the same way on every layout. The sheet holds one ordinary
+`WorkspaceTab`, which is not persisted.
+
+The sheet is modal and covers the whole compact column, header included. It renders in the
+workspace tree rather than through `IsolatedBottomSheetModal`: its panel needs the navigator and
+every workspace provider an ordinary pane has, and a portal drops them
+([floating-panels.md Gotcha 7](floating-panels.md#gotcha-7--a-sheet-cannot-read-context-from-its-call-site)).
+
+A sheet holding unsaved work is never implicitly displaced; only its guarded close removes it.
+Moving its tab to another host remounts the panel, and the file editor keeps unsaved text in the
+mounted view, so the move would drop it. Widening the layout is the known exception: it still moves
+the sheet's tab into a pane, and edits the autosave has not started writing are lost.
+
 ## Explorer sidebar
 
 `packages/app/src/workspace-tabs/explorer-sidebar.ts` owns show, hide, toggle, and view selection.
@@ -62,7 +81,9 @@ lifecycle.
 
 `packages/app/src/workspace-tabs/open-beside.ts` owns content opened beside the user's work. The
 layout store remembers one ordinary pane per workspace. The first side open creates a full-height
-right split around the workspace root; later side opens reuse it.
+right split around the workspace root; later side opens reuse it. Only the `split-panes` layout
+mode opens beside. Compact and single-pane layouts (touch tablets) open the same target in the main
+pane, so callers never check whether splits render.
 
 Removing a pane clears its remembered id; a later side open creates a new pane. The last visible
 ordinary pane stays when its final tab closes and shows the New launcher. An empty workspace does

docs/floating-panels.md

@@ -304,7 +304,9 @@ track phase.
 
 All sheets use the app-wide Gorhom modal provider through `IsolatedBottomSheetModal`. This
 includes native centered dialogs: separate React Native Modals cannot present a root-owned
-sibling dialog over an existing iOS presenter.
+sibling dialog over an existing iOS presenter. The one exception is the
+[workspace sheet](explorer-sidebar.md#workspace-sheet), an in-tree Gorhom `BottomSheet` that hosts a
+whole workspace panel.
 
 The wrapper must give Gorhom an object ref. Gorhom accepts a callback ref at the React boundary
 but stores it in the modal queue and later reads `.current`; provider dismissal, replacement,

docs/glossary.md

@@ -21,6 +21,7 @@ Authoritative terminology. UI label wins. Don't invent synonyms; use what's here
 - **Repository / Remote** — Internal Git observations. They may produce mutable kind, branch, and project-grouping metadata but never the host-local project ID, root, display name, or workspace membership. No UI label.
 - **Sidebar items** — The configurable entries of the app sidebar, in two sections. Header: the rows at the head of the sidebar — New workspace, History, Search, Schedules, and plugin header items. Footer: the full-width rows between Add project and the footer icon row — Usage and plugin footer items. Add project and the icon row (Hosts, Help and support, Settings) are fixed and are not sidebar items. A plugin item may render several rows, such as one per bot; it is still one item that moves and hides as a block. Each section's order and visibility is one per-device preference (`AppSettings.sidebarNavItems`, `AppSettings.sidebarFooterItems`); every item may be off. UI: the Header and Footer cards on the Sidebar settings page. Code: `packages/app/src/sidebar-nav/model.ts` resolves a section, and the sidebar (`components/sidebar/sidebar-nav-rows.tsx`, the footer in `components/left-sidebar.tsx`) and the settings page (`screens/settings/sidebar/sidebar-nav-section.tsx`) read it through `useSidebarNavItems(section)`. Never hardcode one of these entries or add a second place that decides which ones show. Don't confuse with **Explorer sidebar**.
 - **Explorer sidebar** — The dedicated navigation dock containing the Files and Changes trees. UI: "Explorer sidebar". Code: `packages/app/src/workspace-tabs/explorer-sidebar.ts`; persisted pane identifiers retain the `explorer` spelling. See [explorer-sidebar.md](explorer-sidebar.md).
+- **Workspace sheet** — The compact sheet that holds one file or subagent tab over the workspace without adding it to the tab switcher. UI action: "Open as tab". Code: `sheetByWorkspace` in `packages/app/src/stores/workspace-layout-store.ts`, `WorkspaceSheetHost` (`packages/app/src/screens/workspace/workspace-sheet-host.tsx`). See [explorer-sidebar.md](explorer-sidebar.md#workspace-sheet).
 - **Side pane** — An ordinary right-side workspace pane used for content opened beside the user's work. It closes and participates in workspace splits like every other main-host pane. See [explorer-sidebar.md](explorer-sidebar.md#side-pane). Forbidden: "Side panel".
 - **Directory-backed surface** — A right-sidebar surface whose content is determined by the workspace's `cwd`, so two workspaces on the same directory see identical content: git diff/status, forge change-request info, file preview/explorer contents. Keyed by `(serverId, cwd)`, never `workspaceId`. See [architecture.md](architecture.md#right-sidebar-boundary-directory-backed-vs-workspace-owned).
 - **Workspace-owned state** — Per-workspace state that never leaks to a same-`cwd` sibling: tabs, agents, terminals, panes, title, plus review drafts, diff-mode overrides, composer attachments, and file-explorer open/expand state. Keyed by `workspaceId` (`cwd` only as a fallback for old payloads). See [architecture.md](architecture.md#right-sidebar-boundary-directory-backed-vs-workspace-owned).

packages/app/e2e/browser/compact-workspace-sheet.spec.ts

@@ -0,0 +1,106 @@
+import { readFile, writeFile } from "node:fs/promises";
+import path from "node:path";
+import { expect, test } from "../support/fixtures";
+import { expectComposerVisible } from "../support/helpers/composer";
+import { installDaemonWebSocketGate } from "../support/helpers/daemon-websocket-gate";
+import { openAgentRoute, seedMockAgentWorkspace } from "../support/helpers/mock-agent";
+import { openSubagentFromCompactTrack, seedSubagent } from "../support/helpers/subagents";
+import {
+  closeWorkspaceSheet,
+  expectOpenTabCount,
+  openWorkspaceSheetAsTab,
+  expectDeclinedDiscardPrompt,
+  expectWorkspaceSheetAtSnap,
+  replaceWorkspaceSheetFileText,
+  selectTabFromSwitcher,
+  tapWorkspaceSheetBackdrop,
+  workspaceSheet,
+} from "../support/helpers/workspace-sheet";
+
+const FILE_LINK = "linked.ts:3";
+
+function promptMentioning(target: string): string {
+  return [
+    "Generate a title and a git branch name for a coding agent from the user prompt and attachments.",
+    "Return JSON only with fields 'title' and 'branch'.",
+    "",
+    "<user-prompt>",
+    `Open \`${target}\` now`,
+    "</user-prompt>",
+  ].join("\n");
+}
+
+test("compact opens files and subagents in a sheet, leaving the tabs for agents", async ({
+  page,
+}) => {
+  await page.setViewportSize({ width: 390, height: 844 });
+  const workspace = await seedMockAgentWorkspace({
+    repoPrefix: "compact-workspace-sheet-",
+    title: "Sheet parent",
+    initialPrompt: promptMentioning(FILE_LINK),
+  });
+  try {
+    const gate = await installDaemonWebSocketGate(page);
+    const linkedPath = path.join(workspace.cwd, "linked.ts");
+    await writeFile(linkedPath, "one\ntwo\nthree\n", "utf8");
+    const child = await seedSubagent(
+      { client: workspace.client, repoPath: workspace.cwd, workspaceId: workspace.workspaceId },
+      { parentId: workspace.agentId, title: "Sheet child" },
+    );
+    await openAgentRoute(page, { workspaceId: workspace.workspaceId, agentId: workspace.agentId });
+    await expectOpenTabCount(page, 1);
+
+    await test.step("a chat file link opens the file in a sheet", async () => {
+      const fileLink = page.getByText(FILE_LINK, { exact: true });
+      await expect(fileLink).toBeVisible({ timeout: 15_000 });
+      await fileLink.click();
+
+      const sheet = workspaceSheet(page, "linked.ts");
+      await expect(sheet).toBeVisible();
+      await expect(sheet.getByText("three")).toBeVisible();
+      await expectWorkspaceSheetAtSnap(page, "linked.ts");
+      await expect(sheet.getByRole("button", { name: "Open as tab" })).toHaveCount(0);
+      await expectOpenTabCount(page, 1);
+      await closeWorkspaceSheet(page, "linked.ts");
+    });
+
+    await test.step("a subagent opens in a sheet without its composer", async () => {
+      await openSubagentFromCompactTrack(page, child.id);
+
+      const sheet = workspaceSheet(page, child.title);
+      await expect(sheet).toBeVisible();
+      await expectWorkspaceSheetAtSnap(page, child.title);
+      await expect(sheet.getByRole("textbox", { name: "Message agent..." })).toHaveCount(0);
+      await expectOpenTabCount(page, 1);
+    });
+
+    await test.step("Open as tab promotes the subagent to a full tab", async () => {
+      await openWorkspaceSheetAsTab(page, child.title);
+      await expectOpenTabCount(page, 2);
+      await expectComposerVisible(page);
+    });
+
+    await test.step("a sheet with unsaved edits closes only through the guarded close", async () => {
+      await selectTabFromSwitcher(page, "Sheet parent");
+      await page.getByText(FILE_LINK, { exact: true }).click();
+      const sheet = workspaceSheet(page, "linked.ts");
+      await expect(sheet).toBeVisible();
+      // Hold the autosave so the edit stays unsaved.
+      gate.holdNextClientRequest("fs.file.write.request");
+      await replaceWorkspaceSheetFileText(page, "linked.ts", "edited in the sheet");
+      await gate.waitForHeldClientRequest();
+
+      await expectDeclinedDiscardPrompt(page, () => tapWorkspaceSheetBackdrop(page));
+      await expect(sheet.locator(".cm-content")).toHaveText("edited in the sheet");
+      await expectDeclinedDiscardPrompt(page, () =>
+        sheet.getByRole("button", { name: "Close" }).click(),
+      );
+      await expect(sheet.locator(".cm-content")).toHaveText("edited in the sheet");
+
+      gate.releaseHeldClientRequest();
+      await expect.poll(() => readFile(linkedPath, "utf8")).toBe("edited in the sheet");
+    });
+  } finally {
+    await workspace.cleanup();
+  }
+});

packages/app/e2e/support/helpers/subagents.ts

@@ -49,31 +49,37 @@ export async function seedParentWithSubagent(
     modeId: "load-test",
     model: "ten-second-stream",
   });
-  const child = await workspace.client.createAgent({
+  const child = await seedSubagent(workspace, { parentId: parent.id, title: input.childTitle });
-    provider: "mock",
-    cwd: workspace.repoPath,
-    workspaceId: workspace.workspaceId,
-    title: input.childTitle,
-    modeId: "load-test",
-    model: "ten-second-stream",
-    labels: {
-      [PARENT_AGENT_ID_LABEL]: parent.id,
-    },
-  });
 
   return {
     parent: {
       id: parent.id,
       title: input.parentTitle,
     },
-    child: {
+    child,
-      id: child.id,
-      title: input.childTitle,
-    },
     workspaceId: workspace.workspaceId,
   };
 }
 
+/** Seeds an idle Paseo subagent of an existing agent in the same workspace. */
+export async function seedSubagent(
+  workspace: Pick<SeededWorkspace, "client" | "repoPath" | "workspaceId">,
+  input: { parentId: string; title: string },
+): Promise<{ id: string; title: string }> {
+  const child = await workspace.client.createAgent({
+    provider: "mock",
+    cwd: workspace.repoPath,
+    workspaceId: workspace.workspaceId,
+    title: input.title,
+    modeId: "load-test",
+    model: "ten-second-stream",
+    labels: {
+      [PARENT_AGENT_ID_LABEL]: input.parentId,
+    },
+  });
+  return { id: child.id, title: input.title };
+}
+
 export async function seedParentWithCrossWorkspaceSubagent(
   workspace: Pick<SeededWorkspace, "client" | "repoPath" | "workspaceId" | "projectId">,
   input: { parentTitle: string; childTitle: string },
@@ -141,6 +147,12 @@ export async function openSubagentsTrack(page: Page): Promise<void> {
   await expect(panel).toBeVisible({ timeout: 30_000 });
 }
 
+/** On compact the track opens as a sheet, which closes once a row is chosen. */
+export async function openSubagentFromCompactTrack(page: Page, childId: string): Promise<void> {
+  await page.getByTestId("subagents-track-header").click();
+  await page.getByTestId(`subagents-track-row-${childId}`).click();
+}
+
 export async function expectSubagentRowVisible(page: Page, childId: string): Promise<void> {
   await expect(page.getByTestId(`subagents-track-row-${childId}`)).toBeVisible({
     timeout: 30_000,

packages/app/e2e/support/helpers/workspace-sheet.ts

@@ -0,0 +1,94 @@
+import { expect, type Locator, type Page } from "@playwright/test";
+
+/** The compact workspace sheet, named after the tab it hosts. */
+export function workspaceSheet(page: Page, label: string): Locator {
+  return page.getByRole("dialog", { name: label });
+}
+
+/** Sheet height as a fraction of the viewport; mirrors the host's 92% snap point. */
+const SHEET_SNAP_FRACTION = 0.92;
+/** The dialog starts below the sheet's drag handle. */
+const SHEET_HANDLE_MAX_HEIGHT = 32;
+
+/**
+ * The sheet rests at its snap point, docked to the bottom edge. A sheet pushed up past it
+ * covers the workspace header; a sheet sunk below it hides content.
+ */
+export async function expectWorkspaceSheetAtSnap(page: Page, label: string): Promise<void> {
+  const viewportHeight = page.viewportSize()?.height ?? 0;
+  const snapTop = Math.round(viewportHeight * (1 - SHEET_SNAP_FRACTION));
+  const sheet = workspaceSheet(page, label);
+  await expect
+    .poll(async () => {
+      const box = await sheet.boundingBox();
+      if (!box) return null;
+      const top = Math.round(box.y);
+      return {
+        dockedToBottom: Math.round(box.y + box.height) === viewportHeight,
+        topAtSnap: top >= snapTop && top <= snapTop + SHEET_HANDLE_MAX_HEIGHT,
+      };
+    })
+    .toEqual({ dockedToBottom: true, topAtSnap: true });
+}
+
+export async function expectOpenTabCount(page: Page, count: number): Promise<void> {
+  await expect(
+    page.getByRole("button", { name: `Switch tabs (${count} open)` }).filter({ visible: true }),
+  ).toBeVisible();
+}
+
+export async function closeWorkspaceSheet(page: Page, label: string): Promise<void> {
+  const sheet = workspaceSheet(page, label);
+  await sheet.getByRole("button", { name: "Close" }).click();
+  await expect(sheet).toHaveCount(0);
+}
+
+export async function openWorkspaceSheetAsTab(page: Page, label: string): Promise<void> {
+  const sheet = workspaceSheet(page, label);
+  await sheet.getByRole("button", { name: "Open as tab" }).click();
+  await expect(sheet).toHaveCount(0);
+}
+
+export async function replaceWorkspaceSheetFileText(
+  page: Page,
+  label: string,
+  content: string,
+): Promise<void> {
+  const editor = workspaceSheet(page, label)
+    .getByTestId("file-source-editor")
+    .locator(".cm-content");
+  await editor.click();
+  await editor.press("ControlOrMeta+A");
+  await editor.pressSequentially(content);
+}
+
+export async function selectTabFromSwitcher(page: Page, label: string): Promise<void> {
+  await page
+    .getByRole("button", { name: /Switch tabs/ })
+    .filter({ visible: true })
+    .click();
+  const option = page.getByText(label, { exact: true }).filter({ visible: true }).last();
+  await expect(option).toBeVisible();
+  await option.click();
+  await expect(page.getByRole("button", { name: "Bottom sheet backdrop" })).toHaveCount(0);
+}
+
+export async function tapWorkspaceSheetBackdrop(page: Page): Promise<void> {
+  await page
+    .getByRole("button", { name: "Bottom sheet backdrop" })
+    .click({ position: { x: 10, y: 10 } });
+}
+
+/** Runs a close that hits the unsaved-changes guard, and declines the discard. */
+export async function expectDeclinedDiscardPrompt(
+  page: Page,
+  close: () => Promise<void>,
+): Promise<void> {
+  const prompt = page.waitForEvent("dialog").then(async (dialog) => {
+    const message = dialog.message();
+    await dialog.dismiss();
+    return message;
+  });
+  await close();
+  expect(await prompt).toContain("discard the draft");
+}

packages/app/src/app/_layout.tsx

@@ -116,6 +116,7 @@ import { usePanelStore } from "@/stores/panel-store";
 import { flushDraftPersistStorage } from "@/stores/draft-store";
 import { getNextThemePreference, ICON_STROKE_WIDTH } from "@/styles/theme";
 import { useSessionStore } from "@/stores/session-store";
+import { WorkspaceLayoutModePublisher } from "@/workspace-tabs/layout-mode-publisher";
 import { installWebScrollbarStyles } from "@/styles/install-web-scrollbar-styles";
 import type { HostProfile } from "@/types/host-connection";
 import {
@@ -549,6 +550,7 @@ function AppContainer({ children, chromeEnabled: chromeEnabledOverride }: AppCon
   }
   const workspaceChrome = (
     <View style={rowStyle}>
+      <WorkspaceLayoutModePublisher />
       {!isCompactLayout ? (
         <WindowChromeRegion corners={appChromeLayout.sidebarCorners}>
           {themedSidebarChrome}

packages/app/src/file-pane/editor/view.web.tsx

@@ -165,4 +165,7 @@ const FRAME_STYLE = {
   minHeight: 0,
   minWidth: 0,
 } as const;
-const HOST_STYLE = { flex: 1, minHeight: 0, overflow: "hidden" } as const;
+// CodeMirror's scrollIntoView scrolls every overflowing ancestor and stops at a fixed or sticky
+// one. Sticky (with no offsets it lays out like relative) keeps line reveals inside the editor, so
+// they don't shift the host around it, such as the compact workspace sheet.
+const HOST_STYLE = { flex: 1, minHeight: 0, overflow: "hidden", position: "sticky" } as const;

packages/app/src/i18n/resources/ar.ts

@@ -613,6 +613,9 @@ export const ar: TranslationResources = {
       updateHost: "حدّث المضيف لاستخدام عارض الطرفية الأصلي.",
       unableToSubscribe: "غير قادر على الاشتراك في المحطة",
     },
+    sheet: {
+      openAsTab: "فتح كعلامة تبويب",
+    },
     tabs: {
       loading: "تحميل...",
       modified: "تغييرات غير محفوظة",

packages/app/src/i18n/resources/en.ts

@@ -610,6 +610,9 @@ export const en = {
       updateHost: "Update the host to use the native terminal renderer.",
       unableToSubscribe: "Unable to subscribe to terminal",
     },
+    sheet: {
+      openAsTab: "Open as tab",
+    },
     tabs: {
       loading: "Loading...",
       modified: "Unsaved changes",

packages/app/src/i18n/resources/es.ts

@@ -619,6 +619,9 @@ export const es: TranslationResources = {
       updateHost: "Actualiza el host para usar el renderizador de terminal nativo.",
       unableToSubscribe: "No se puede suscribir al terminal",
     },
+    sheet: {
+      openAsTab: "Abrir como pestaña",
+    },
     tabs: {
       loading: "Cargando...",
       modified: "Cambios sin guardar",

packages/app/src/i18n/resources/fr.ts

@@ -618,6 +618,9 @@ export const fr: TranslationResources = {
       updateHost: "Mettez à jour l’hôte pour utiliser le rendu natif du terminal.",
       unableToSubscribe: "Impossible de s’abonner au terminal",
     },
+    sheet: {
+      openAsTab: "Ouvrir dans un onglet",
+    },
     tabs: {
       loading: "Chargement…",
       modified: "Modifications non enregistrées",

packages/app/src/i18n/resources/ja.ts

@@ -619,6 +619,9 @@ export const ja: TranslationResources = {
       updateHost: "ネイティブターミナルを使用するにはホストを更新してください。",
       unableToSubscribe: "ターミナルに接続できません",
     },
+    sheet: {
+      openAsTab: "タブで開く",
+    },
     tabs: {
       loading: "読み込み中...",
       modified: "未保存の変更",

packages/app/src/i18n/resources/ko.ts

@@ -614,6 +614,9 @@ export const ko: TranslationResources = {
       updateHost: "기본 터미널 렌더러를 사용하도록 호스트를 업데이트합니다.",
       unableToSubscribe: "터미널을 구독할 수 없습니다",
     },
+    sheet: {
+      openAsTab: "탭으로 열기",
+    },
     tabs: {
       loading: "불러오는 중...",
       modified: "저장되지 않은 변경사항",

packages/app/src/i18n/resources/pt-BR.ts

@@ -618,6 +618,9 @@ export const ptBR: TranslationResources = {
       updateHost: "Atualize o host para usar o renderizador de terminal nativo.",
       unableToSubscribe: "Não foi possível assinar o terminal",
     },
+    sheet: {
+      openAsTab: "Abrir como aba",
+    },
     tabs: {
       loading: "Carregando...",
       modified: "Alterações não salvas",

packages/app/src/i18n/resources/ru.ts

@@ -620,6 +620,9 @@ export const ru: TranslationResources = {
       updateHost: "Обновите хост, чтобы использовать нативный рендерер терминала.",
       unableToSubscribe: "Не удалось подключиться к терминалу",
     },
+    sheet: {
+      openAsTab: "Открыть во вкладке",
+    },
     tabs: {
       loading: "Загрузка...",
       modified: "Несохранённые изменения",

packages/app/src/i18n/resources/zh-CN.ts

@@ -613,6 +613,9 @@ export const zhCN: TranslationResources = {
       updateHost: "请更新主机以使用原生终端渲染器。",
       unableToSubscribe: "无法订阅 Terminal",
     },
+    sheet: {
+      openAsTab: "作为标签页打开",
+    },
     tabs: {
       loading: "正在加载...",
       modified: "未保存的更改",

packages/app/src/panels/agent-panel.tsx

@@ -1153,6 +1153,8 @@ const ChatAgentReadyContent = memo(function ChatAgentReadyContent({
   onOpenWorkspaceFile?: (request: WorkspaceFileOpenRequest) => void;
 }) {
   const { t } = useTranslation();
+  // The workspace sheet is for looking; Open as tab brings the agent back with its composer.
+  const showsComposer = usePaneContext().host !== "sheet";
   const subagentRows = useSubagentsForParent({ serverId, parentAgentId: agentId });
   const tasks = useSessionStore((state): TodoEntry[] | undefined =>
     state.sessions[serverId]?.agentTasks.get(agentId),
@@ -1216,7 +1218,7 @@ const ChatAgentReadyContent = memo(function ChatAgentReadyContent({
       composerState,
     ],
   );
-  const composerSection = (
+  const composerSection = showsComposer ? (
     <RenderProfile id={`AgentComposerSection:${agentId}`}>
       <AgentComposerSection
         agentId={agentId}
@@ -1233,7 +1235,7 @@ const ChatAgentReadyContent = memo(function ChatAgentReadyContent({
         onMessageSent={handleMessageSent}
       />
     </RenderProfile>
-  );
+  ) : null;
   const streamContent = (
     <View style={animatedStaticStyles.content}>
       <RenderProfile id={`AgentStreamSection:${agentId}`}>

packages/app/src/panels/agent-tracks.tsx

@@ -3,7 +3,7 @@ import { WorkspaceDiffStatPill } from "@/composer/diff-stat-pill";
 import { useWorkspaceHasDiffStat } from "@/composer/workspace-diff-stat";
 import { AgentTaskList } from "@/composer/task-list";
 import { ComposerTrackBar } from "@/composer/tracks";
-import { supportsDesktopPaneSplits, useIsCompactFormFactor } from "@/constants/layout";
+import { useIsCompactFormFactor } from "@/constants/layout";
 import { usePaneContext } from "@/panels/pane-context";
 import { useSettings } from "@/hooks/use-settings";
 import { PluginComposerPills } from "@/plugins";
@@ -17,7 +17,7 @@ import {
 import { SubagentsTrack } from "@/subagents/track";
 import type { TodoEntry } from "@/types/stream";
 import { navigateToAgent } from "@/utils/navigate-to-agent";
-import { buildWorkspaceTabPersistenceKey } from "@/workspace-tabs/model";
+import { buildWorkspaceTabPersistenceKey, type WorkspaceTabTarget } from "@/workspace-tabs/model";
 import { openPreferredWorkspaceTarget } from "@/workspace-tabs/open-beside";
 import { openComposerChanges } from "@/workspace-tabs/open-supporting-view";
 
@@ -49,10 +49,9 @@ export const AgentTracks = memo(function AgentTracks({
   onArchiveFinished: () => void;
   hasPluginComposerPills: boolean;
 }): ReactElement | null {
-  const { tabId, openTab } = usePaneContext();
+  const { tabId } = usePaneContext();
   const hasWorkspaceDiffStat = useWorkspaceHasDiffStat(serverId, workspaceId);
   const isCompact = useIsCompactFormFactor();
-  const canSplit = supportsDesktopPaneSplits() && !isCompact;
   const openInSidePane = useSettings((settings) => settings.openInSidePane);
   const workspaceKey = buildWorkspaceTabPersistenceKey({ serverId, workspaceId });
   const canDetachSubagents = useSessionStore(
@@ -60,45 +59,36 @@ export const AgentTracks = memo(function AgentTracks({
   );
   const archiveSubagent = useArchiveSubagent({ serverId });
   const detachSubagent = useDetachSubagent({ serverId });
+  const openSubagentTarget = useCallback(
+    (target: WorkspaceTabTarget) => {
+      openPreferredWorkspaceTarget({
+        workspaceKey,
+        target,
+        source: "subagents",
+        preferences: openInSidePane,
+        parentTabId: tabId,
+      });
+    },
+    [openInSidePane, tabId, workspaceKey],
+  );
   const handleOpenSubagent = useCallback(
     (subagentId: string) => {
       const session = useSessionStore.getState().sessions[serverId];
       const agent = session?.agents.get(subagentId) ?? session?.agentDetails.get(subagentId);
+      // A subagent in another workspace lives in that workspace's tabs.
       if (agent?.workspaceId && agent.workspaceId !== workspaceId) {
         navigateToAgent({ serverId, agentId: subagentId });
         return;
       }
-      if (canSplit && workspaceKey) {
+      openSubagentTarget({ kind: "agent", agentId: subagentId });
-        openPreferredWorkspaceTarget({
-          isCompact,
-          workspaceKey,
-          target: { kind: "agent", agentId: subagentId },
-          source: "subagents",
-          preferences: openInSidePane,
-          parentTabId: tabId,
-        });
-        return;
-      }
-      navigateToAgent({ serverId, agentId: subagentId });
     },
-    [canSplit, isCompact, openInSidePane, serverId, tabId, workspaceId, workspaceKey],
+    [openSubagentTarget, serverId, workspaceId],
   );
   const handleOpenProviderSubagent = useCallback(
     (parentAgentId: string, subagentId: string) => {
-      if (canSplit && workspaceKey) {
+      openSubagentTarget({ kind: "provider_subagent", parentAgentId, subagentId });
-        openPreferredWorkspaceTarget({
-          isCompact,
-          workspaceKey,
-          target: { kind: "provider_subagent", parentAgentId, subagentId },
-          source: "subagents",
-          preferences: openInSidePane,
-          parentTabId: tabId,
-        });
-        return;
-      }
-      openTab({ kind: "provider_subagent", parentAgentId, subagentId });
     },
-    [canSplit, isCompact, openInSidePane, openTab, tabId, workspaceKey],
+    [openSubagentTarget],
   );
   const handleOpenChanges = useCallback(() => {
     if (!workspaceKey) {

packages/app/src/panels/panel-instance-attributes.ts

@@ -1,5 +1,6 @@
 import { useCallback, useEffect, useMemo, useSyncExternalStore } from "react";
 import { usePaneContext } from "@/panels/pane-context";
+import { buildWorkspaceTabPersistenceKey } from "@/workspace-tabs/model";
 
 export interface PanelInstanceIdentity {
   serverId: string;
@@ -18,8 +19,20 @@ const listenersByPanel = new Map<string, Set<() => void>>();
 const allListeners = new Set<() => void>();
 let attributesRevision = 0;
 
+function buildWorkspacePanelKey(workspaceKey: string, tabId: string): string {
+  return `${workspaceKey}:${tabId}`;
+}
+
 export function buildPanelInstanceKey(identity: PanelInstanceIdentity): string {
-  return `${identity.serverId}:${identity.workspaceId}:${identity.tabId}`;
+  return buildWorkspacePanelKey(buildWorkspaceTabPersistenceKey(identity) ?? "", identity.tabId);
+}
+
+/** Looks a panel up by the workspace key the layout store uses. */
+export function getWorkspacePanelAttributes(
+  workspaceKey: string,
+  tabId: string,
+): PanelInstanceAttributes {
+  return attributesByPanel.get(buildWorkspacePanelKey(workspaceKey, tabId)) ?? DEFAULT_ATTRIBUTES;
 }
 
 export function getPanelInstanceAttributes(

packages/app/src/panels/panel-manifest.test.ts

@@ -18,6 +18,14 @@ describe("panel manifest", () => {
     expect(panelSupportsHost("setup", "explorer")).toBe(false);
   });
 
+  it("supports the sheet exactly for kinds with a compact sheet rule", () => {
+    expect(panelSupportsHost("file", "sheet")).toBe(true);
+    expect(panelSupportsHost("agent", "sheet")).toBe(true);
+    expect(panelSupportsHost("provider_subagent", "sheet")).toBe(true);
+    expect(panelSupportsHost("terminal", "sheet")).toBe(false);
+    expect(panelSupportsHost("plugin", "sheet")).toBe(false);
+  });
+
   it("lets navigation tabs hide their close button without losing their close action", () => {
     expect(getPanelManifest("files").showCloseButton).toBe(false);
     expect(getPanelManifest("changes_tree").showCloseButton).toBe(false);

packages/app/src/panels/panel-manifest.ts

@@ -1,12 +1,38 @@
 import type { WorkspaceTabTarget } from "@/workspace-tabs/model";
 
-export type PaneHost = "main" | "explorer";
+/** Where a tab renders: a split pane, the Explorer dock, or the compact workspace sheet. */
+export type PaneHost = "main" | "explorer" | "sheet";
+
+/** How a target behaves while it is shown in the compact workspace sheet. */
+export interface CompactSheetPolicy {
+  /** The sheet offers Open as tab, which moves the same tab into the focused pane. */
+  openAsTab: boolean;
+}
+
+/** Facts about a target that live outside the layout, resolved lazily by the workspace. */
+export interface PanelTargetFacts {
+  isWorkspaceRootAgent(agentId: string): boolean;
+}
+
+type CompactSheetRule<K extends WorkspaceTabTarget["kind"]> =
+  | CompactSheetPolicy
+  | null
+  | ((
+      target: Extract<WorkspaceTabTarget, { kind: K }>,
+      facts: PanelTargetFacts,
+    ) => CompactSheetPolicy | null);
 
 export interface PanelManifest<K extends WorkspaceTabTarget["kind"] = WorkspaceTabTarget["kind"]> {
   kind: K;
-  supportedHosts: readonly PaneHost[];
+  /** The sheet is never listed: a kind renders there exactly when it has a `compactSheet` rule. */
+  supportedHosts: readonly Exclude<PaneHost, "sheet">[];
   showCloseButton: boolean;
   singleton: boolean;
+  /**
+   * On compact layouts, revealing this target opens it in the workspace sheet over the current
+   * tab instead of adding a tab. `null` keeps it in the tab strip.
+   */
+  compactSheet: CompactSheetRule<K>;
   resourceKey(target: Extract<WorkspaceTabTarget, { kind: K }>): string;
 }
 
@@ -20,6 +46,7 @@ const manifests = {
     supportedHosts: ["main", "explorer"],
     showCloseButton: true,
     singleton: false,
+    compactSheet: null,
     resourceKey: () => "new_tab",
   },
   draft: {
@@ -27,6 +54,7 @@ const manifests = {
     supportedHosts: ["main", "explorer"],
     showCloseButton: true,
     singleton: false,
+    compactSheet: null,
     resourceKey: (target) => target.draftId,
   },
   agent: {
@@ -34,6 +62,8 @@ const manifests = {
     supportedHosts: ["main", "explorer"],
     showCloseButton: true,
     singleton: false,
+    compactSheet: (target, facts) =>
+      facts.isWorkspaceRootAgent(target.agentId) ? null : { openAsTab: true },
     resourceKey: (target) => target.agentId,
   },
   provider_subagent: {
@@ -41,6 +71,7 @@ const manifests = {
     supportedHosts: ["main", "explorer"],
     showCloseButton: true,
     singleton: false,
+    compactSheet: { openAsTab: false },
     resourceKey: (target) => `${target.parentAgentId}:${target.subagentId}`,
   },
   terminal: {
@@ -48,6 +79,7 @@ const manifests = {
     supportedHosts: ["main", "explorer"],
     showCloseButton: true,
     singleton: false,
+    compactSheet: null,
     resourceKey: (target) => target.terminalId,
   },
   browser: {
@@ -55,6 +87,7 @@ const manifests = {
     supportedHosts: ["main"],
     showCloseButton: true,
     singleton: false,
+    compactSheet: null,
     resourceKey: (target) => target.browserId,
   },
   changes_tree: {
@@ -62,6 +95,7 @@ const manifests = {
     supportedHosts: ["explorer"],
     showCloseButton: false,
     singleton: true,
+    compactSheet: null,
     resourceKey: () => "changes_tree",
   },
   files: {
@@ -69,6 +103,7 @@ const manifests = {
     supportedHosts: ["explorer"],
     showCloseButton: false,
     singleton: true,
+    compactSheet: null,
     resourceKey: () => "files",
   },
   pull_request: {
@@ -76,6 +111,7 @@ const manifests = {
     supportedHosts: ["main", "explorer"],
     showCloseButton: true,
     singleton: false,
+    compactSheet: null,
     resourceKey: () => "pull_request",
   },
   file: {
@@ -83,6 +119,7 @@ const manifests = {
     supportedHosts: ["main", "explorer"],
     showCloseButton: true,
     singleton: false,
+    compactSheet: { openAsTab: false },
     resourceKey: (target) => target.path,
   },
   working_diff: {
@@ -90,6 +127,7 @@ const manifests = {
     supportedHosts: ["main", "explorer"],
     showCloseButton: true,
     singleton: false,
+    compactSheet: null,
     resourceKey: () => "working_diff",
   },
   plugin: {
@@ -98,6 +136,7 @@ const manifests = {
     supportedHosts: ["main", "explorer"],
     showCloseButton: true,
     singleton: false,
+    compactSheet: null,
     resourceKey: (target) =>
       target.context === "agent"
         ? `${target.pluginId}:${target.panelId}:agent:${target.agentId}`
@@ -108,6 +147,7 @@ const manifests = {
     supportedHosts: ["main"],
     showCloseButton: true,
     singleton: false,
+    compactSheet: null,
     resourceKey: (target) => target.workspaceId,
   },
   commit_diff: {
@@ -115,6 +155,7 @@ const manifests = {
     supportedHosts: ["main", "explorer"],
     showCloseButton: true,
     singleton: false,
+    compactSheet: null,
     resourceKey: (target) => target.sha,
   },
 } satisfies PanelManifestByKind;
@@ -124,7 +165,17 @@ export function getPanelManifest<K extends WorkspaceTabTarget["kind"]>(kind: K):
 }
 
 export function panelSupportsHost(kind: WorkspaceTabTarget["kind"], host: PaneHost): boolean {
-  return getPanelManifest(kind).supportedHosts.includes(host);
+  const manifest = getPanelManifest(kind);
+  if (host === "sheet") return manifest.compactSheet !== null;
+  return manifest.supportedHosts.includes(host);
+}
+
+export function panelCompactSheetPolicy(
+  target: WorkspaceTabTarget,
+  facts: PanelTargetFacts,
+): CompactSheetPolicy | null {
+  const rule = getPanelManifest(target.kind).compactSheet as CompactSheetRule<typeof target.kind>;
+  return typeof rule === "function" ? rule(target as never, facts) : rule;
 }
 
 export function panelCanLaunchInPane(

packages/app/src/plugins/workspace-panels/locations.ts

@@ -50,7 +50,7 @@ export function panelTargetSupportsHost(
   target: WorkspaceTabTarget,
   host: PaneHost,
 ): boolean {
-  if (target.kind !== "plugin") return panelSupportsHost(target.kind, host);
+  if (target.kind !== "plugin" || host === "sheet") return panelSupportsHost(target.kind, host);
   const panel = findPluginPanel(serverId, target);
   // Preserve a persisted unavailable panel where the user left it until its plugin loads again.
   if (!panel) return true;

packages/app/src/screens/workspace/visible-agent-ids.test.ts

@@ -110,3 +110,25 @@ test("pane retargeting replaces the viewed agent and duplicate panes collapse to
     }),
   }).toEqual({ duplicate: ["agent-a"], retargeted: ["agent-a", "agent-b"] });
 });
+
+test("the agent shown in the workspace sheet is viewed alongside the pane under it", () => {
+  const layout: WorkspaceLayout = {
+    focusedPaneId: "main",
+    root: { kind: "pane", pane: { id: "main", tabIds: ["a"], focusedTabId: "a" } },
+  };
+  const tabs: WorkspaceTab[] = [
+    { tabId: "a", target: { kind: "agent", agentId: "agent-a" }, createdAt: 1 },
+  ];
+  const sheetTab: WorkspaceTab = {
+    tabId: "sheet",
+    target: { kind: "agent", agentId: "agent-child" },
+    createdAt: 2,
+  };
+
+  expect(
+    selectVisibleAgentIds({ layout, tabs, sheetTab, routeFocused: true, focusedPaneOnly: true }),
+  ).toEqual(["agent-a", "agent-child"]);
+  expect(
+    selectVisibleAgentIds({ layout, tabs, sheetTab, routeFocused: false, focusedPaneOnly: true }),
+  ).toEqual([]);
+});

packages/app/src/screens/workspace/visible-agent-ids.ts

@@ -6,6 +6,8 @@ import { deriveWorkspacePaneState } from "./workspace-pane-state";
 export function selectVisibleAgentIds(input: {
   layout: WorkspaceLayout | null;
   tabs: WorkspaceTab[];
+  /** The workspace sheet's tab, shown over the panes. */
+  sheetTab?: WorkspaceTab | null;
   routeFocused: boolean;
   focusedPaneOnly: boolean;
 }): string[] {
@@ -16,20 +18,21 @@ export function selectVisibleAgentIds(input: {
     ? collectAllPanes(input.layout.root).filter((pane) => pane.id === input.layout?.focusedPaneId)
     : collectAllPanes(input.layout.root);
 
+  const targets = panes.flatMap((pane) => {
+    const target = deriveWorkspacePaneState({ pane, tabs: input.tabs }).activeTab?.descriptor
+      .target;
+    return target ? [target] : [];
+  });
+  if (input.sheetTab) targets.push(input.sheetTab.target);
   return [
-    ...new Set(
+    ...new Set(targets.flatMap((target) => (target.kind === "agent" ? [target.agentId] : []))),
-      panes.flatMap((pane) => {
-        const target = deriveWorkspacePaneState({ pane, tabs: input.tabs }).activeTab?.descriptor
-          .target;
-        return target?.kind === "agent" ? [target.agentId] : [];
-      }),
-    ),
   ].sort();
 }
 
 export function useVisibleAgentIds(input: {
   layout: WorkspaceLayout | null;
   tabs: WorkspaceTab[];
+  sheetTab: WorkspaceTab | null;
   routeFocused: boolean;
   focusedPaneOnly: boolean;
 }): string[] {

packages/app/src/screens/workspace/workspace-screen.tsx

@@ -73,6 +73,7 @@ import {
   getFocusedBrowserId,
   FOCUSED_PANE_PLACEMENT,
   selectExplorerSidebarPaneId,
+  selectPaneFocusSuspended,
   type WorkspaceLayout,
   type WorkspaceTabPlacement,
   useWorkspaceLayoutStore,
@@ -160,6 +161,8 @@ import {
   type WorkspacePaneContentModel,
 } from "@/screens/workspace/workspace-pane-content";
 import { useMountedTabSet } from "@/screens/workspace/use-mounted-tab-set";
+import { WorkspaceSheetHost } from "@/screens/workspace/workspace-sheet-host";
+import type { PaneHost } from "@/panels/panel-manifest";
 import { WorkspaceFocusProvider } from "@/workspace/focus";
 import { DiffDocumentWorkspaceCacheProvider } from "@/git/diff-document/workspace-cache";
 import type { NewTabSelection } from "@/workspace-tabs/new-tab";
@@ -751,22 +754,20 @@ function WorkspaceContentProviders({
   );
 }
 
-function WorkspacePanelContent({
+function WorkspacePanelProviders({
   launcher,
-  content,
+  children,
   tabIds,
   workspaceKey,
 }: {
   workspaceKey: string;
   launcher: NewTabLauncher;
-  content: ReactNode;
+  children: ReactNode;
   tabIds: readonly string[];
 }) {
   return (
     <WorkspaceComposerEditingSessionsProvider workspaceKey={workspaceKey} tabIds={tabIds}>
-      <NewTabLauncherProvider value={launcher}>
+      <NewTabLauncherProvider value={launcher}>{children}</NewTabLauncherProvider>
-        <View style={styles.content}>{content}</View>
-      </NewTabLauncherProvider>
     </WorkspaceComposerEditingSessionsProvider>
   );
 }
@@ -1267,6 +1268,9 @@ function WorkspaceScreenContent({
   const explorerSidebarPaneId = useWorkspaceLayoutStore((state) =>
     persistenceKey ? selectExplorerSidebarPaneId(state, persistenceKey) : null,
   );
+  const paneFocusSuspended = useWorkspaceLayoutStore((state) =>
+    persistenceKey ? selectPaneFocusSuspended(state, persistenceKey) : false,
+  );
   const lastMainPaneRef = useLastMainPane({
     workspaceKey: persistenceKey,
     layout: workspaceLayout,
@@ -1305,6 +1309,9 @@ function WorkspaceScreenContent({
   const focusWorkspaceTab = useWorkspaceLayoutStore((state) => state.focusTab);
   const selectWorkspaceTabInPane = useWorkspaceLayoutStore((state) => state.selectTabInPane);
   const closeWorkspaceTab = useWorkspaceLayoutStore((state) => state.closeTab);
+  const sheetTab = useWorkspaceLayoutStore((state) =>
+    persistenceKey ? (state.sheetByWorkspace[persistenceKey]?.tab ?? null) : null,
+  );
   const unpinWorkspaceAgent = useWorkspaceLayoutStore((state) => state.unpinAgent);
   const hideWorkspaceAgent = useWorkspaceLayoutStore((state) => state.hideAgent);
   const setWorkspaceTabState = useWorkspaceLayoutStore((state) => state.setTabState);
@@ -1377,6 +1384,7 @@ function WorkspaceScreenContent({
   const visibleAgentIds = useVisibleAgentIds({
     layout: workspaceLayout,
     tabs: uiTabs,
+    sheetTab,
     routeFocused: isRouteFocused,
     focusedPaneOnly: syncFocusedPaneOnly,
   });
@@ -1678,7 +1686,6 @@ function WorkspaceScreenContent({
       }
 
       const tabId = openPreferredWorkspaceTarget({
-        isCompact: isMobile,
         workspaceKey: persistenceKey,
         target: createWorkspaceFileTabTarget(location),
         source: "chatFiles",
@@ -2062,6 +2069,12 @@ function WorkspaceScreenContent({
 
   const handleCloseTabById = useCallback(
     async (tabId: string) => {
+      if (persistenceKey && sheetTab?.tabId === tabId) {
+        if (await confirmDiscardModifiedTab(tabId)) {
+          closeWorkspaceTab(persistenceKey, tabId);
+        }
+        return;
+      }
       const tab = allTabDescriptorsById.get(tabId);
       if (!tab) {
         return;
@@ -2081,10 +2094,13 @@ function WorkspaceScreenContent({
     },
     [
       allTabDescriptorsById,
+      closeWorkspaceTab,
       confirmDiscardModifiedTab,
       handleCloseAgentTab,
       handleClosePassiveTab,
       handleCloseTerminalTab,
+      persistenceKey,
+      sheetTab,
     ],
   );
 
@@ -2918,17 +2934,19 @@ function WorkspaceScreenContent({
       tab: WorkspaceTabDescriptor;
       paneId?: string | null;
       focusPaneBeforeOpen?: boolean;
+      host?: PaneHost;
     }) =>
       buildWorkspacePaneContentModel({
         tab: input.tab,
         normalizedServerId,
         normalizedWorkspaceId,
         host:
-          canRenderDesktopPaneSplits &&
+          input.host ??
+          (canRenderDesktopPaneSplits &&
           input.paneId !== null &&
           input.paneId === explorerSidebarPaneId
             ? "explorer"
-            : "main",
+            : "main"),
         fileNavigationRevision: fileNavigationRevisionByTabId[input.tab.tabId] ?? 0,
         onOpenTab: (target) => {
           if (!persistenceKey) {
@@ -2947,7 +2965,6 @@ function WorkspaceScreenContent({
         onOpenPreferredTarget: (target, source) => {
           if (!persistenceKey) return;
           const tabId = openPreferredWorkspacePreview({
-            isCompact: isMobile,
             workspaceKey: persistenceKey,
             serverId: normalizedServerId,
             workspaceId: normalizedWorkspaceId,
@@ -3009,7 +3026,6 @@ function WorkspaceScreenContent({
       canRenderDesktopPaneSplits,
       openImportSheet,
       openInSidePane,
-      isMobile,
       requestFileNavigation,
       revealWorkspaceChildTab,
       persistenceKey,
@@ -3053,6 +3069,17 @@ function WorkspaceScreenContent({
     },
     [buildPaneContentModel],
   );
+  const buildSheetPaneContentModel = useCallback(
+    function buildSheetPaneContentModel(tab: WorkspaceTabDescriptor) {
+      return buildPaneContentModel({
+        tab,
+        paneId: null,
+        focusPaneBeforeOpen: false,
+        host: "sheet",
+      });
+    },
+    [buildPaneContentModel],
+  );
   const handleFocusPane = useStableEvent(function handleFocusPane(paneId: string) {
     if (!persistenceKey || paneFocusSuppressedRef.current) {
       return;
@@ -3068,7 +3095,8 @@ function WorkspaceScreenContent({
     focusedPaneTabDescriptorMap,
     isRouteFocused,
     focusedPaneId,
-    paneFocusSuspended: Boolean(unfocusedPaneId),
+    // The sheet owns input while it covers the tab.
+    paneFocusSuspended,
     onFocusPane: handleFocusPane,
     buildMobilePaneContentModel,
   });
@@ -3495,28 +3523,42 @@ function WorkspaceScreenContent({
   const desktopContent = desktopSplitContent ?? content;
   const rendersDesktopSplitContent = !isMobile && desktopSplitContent !== null;
 
-  const workspacePanelContent = (
+  const workspacePanelTabIds = useMemo(
-    <WorkspacePanelContent
+    () => [...uiTabs.map((tab) => tab.tabId), ...(sheetTab ? [sheetTab.tabId] : [])],
-      launcher={newTabLauncher}
+    [uiTabs, sheetTab],
-      workspaceKey={`${normalizedServerId}:${normalizedWorkspaceId}`}
-      tabIds={uiTabs.map((tab) => tab.tabId)}
-      content={isMobile ? content : desktopContent}
-    />
   );
+  const workspacePanelKey = `${normalizedServerId}:${normalizedWorkspaceId}`;
 
+  // The sheet is the column's last child, so it covers the compact header as a modal.
   const workspaceCenterColumn = isMobile ? (
-    <View style={styles.centerColumn}>
+    <WorkspacePanelProviders
-      <FloatingHeaderLayout
+      launcher={newTabLauncher}
-        header={compactWorkspaceHeader}
+      workspaceKey={workspacePanelKey}
-        floating={floatsHeaderOverContent(activeTabDescriptor)}
+      tabIds={workspacePanelTabIds}
-      >
+    >
-        {(topOverlayInset) => (
+      <View style={styles.centerColumn}>
-          <StreamTopOverlayProvider value={topOverlayInset}>
+        <FloatingHeaderLayout
-            {workspacePanelContent}
+          header={compactWorkspaceHeader}
-          </StreamTopOverlayProvider>
+          floating={floatsHeaderOverContent(activeTabDescriptor)}
-        )}
+        >
-      </FloatingHeaderLayout>
+          {(topOverlayInset) => (
-    </View>
+            <StreamTopOverlayProvider value={topOverlayInset}>
+              <View style={styles.content}>{content}</View>
+            </StreamTopOverlayProvider>
+          )}
+        </FloatingHeaderLayout>
+        <StreamTopOverlayProvider value={0}>
+          <WorkspaceSheetHost
+            workspaceKey={persistenceKey}
+            serverId={normalizedServerId}
+            workspaceId={normalizedWorkspaceId}
+            isWorkspaceFocused={isRouteFocused}
+            buildContent={buildSheetPaneContentModel}
+            onCloseTab={handleCloseTabById}
+          />
+        </StreamTopOverlayProvider>
+      </View>
+    </WorkspacePanelProviders>
   ) : (
     <View style={styles.centerColumn}>
       {rendersDesktopSplitContent ? null : renderWorkspaceScreenHeader()}
@@ -3549,7 +3591,15 @@ function WorkspaceScreenContent({
         </NewTabLauncherProvider>
       ) : null}
 
-      <View style={styles.centerContent}>{workspacePanelContent}</View>
+      <View style={styles.centerContent}>
+        <WorkspacePanelProviders
+          launcher={newTabLauncher}
+          workspaceKey={workspacePanelKey}
+          tabIds={workspacePanelTabIds}
+        >
+          <View style={styles.content}>{desktopContent}</View>
+        </WorkspacePanelProviders>
+      </View>
     </View>
   );
 

packages/app/src/screens/workspace/workspace-sheet-host.tsx

@@ -0,0 +1,245 @@
+import BottomSheet, {
+  BottomSheetBackdrop,
+  type BottomSheetBackdropProps,
+} from "@gorhom/bottom-sheet";
+import { useIsFocused } from "@react-navigation/native";
+import { type ComponentType, useCallback, useEffect, useMemo, useRef } from "react";
+import { BackHandler, View } from "react-native";
+import { useTranslation } from "react-i18next";
+import { useSafeAreaInsets } from "react-native-safe-area-context";
+import { StyleSheet, withUnistyles } from "react-native-unistyles";
+import { SheetHeaderView } from "@/components/adaptive-modal-sheet";
+import { Button } from "@/components/ui/button";
+import { isNative } from "@/constants/platform";
+import type { PanelIconProps } from "@/panels/panel-registry";
+import { getPanelRegistration } from "@/panels/panel-registry";
+import { ensurePanelsRegistered } from "@/panels/register-panels";
+import type { WorkspaceTabDescriptor } from "@/screens/workspace/workspace-tabs-types";
+import {
+  WorkspacePaneContent,
+  type WorkspacePaneContentModel,
+} from "@/screens/workspace/workspace-pane-content";
+import { useWorkspaceLayoutStore, type WorkspaceSheet } from "@/stores/workspace-layout-store";
+
+const SNAP_POINTS = ["92%"];
+
+const ThemedBottomSheet = withUnistyles(BottomSheet, (theme) => ({
+  backgroundStyle: {
+    backgroundColor: theme.colors.surface0,
+    borderTopLeftRadius: theme.borderRadius["2xl"],
+    borderTopRightRadius: theme.borderRadius["2xl"],
+  },
+  handleIndicatorStyle: {
+    backgroundColor: theme.colors.palette.zinc[600],
+  },
+}));
+
+function PanelIcon({
+  icon: Icon,
+  size,
+  color,
+}: { icon: ComponentType<PanelIconProps> } & PanelIconProps) {
+  return <Icon size={size} color={color} />;
+}
+
+const ThemedPanelIcon = withUnistyles(PanelIcon, (theme) => ({
+  color: theme.colors.foregroundMuted,
+}));
+
+function renderBackdrop(props: BottomSheetBackdropProps) {
+  return (
+    <BottomSheetBackdrop
+      {...props}
+      appearsOnIndex={0}
+      disappearsOnIndex={-1}
+      opacity={0.45}
+      pressBehavior="close"
+    />
+  );
+}
+
+export interface WorkspaceSheetHostProps {
+  workspaceKey: string | null;
+  serverId: string;
+  workspaceId: string;
+  isWorkspaceFocused: boolean;
+  buildContent: (tab: WorkspaceTabDescriptor) => WorkspacePaneContentModel;
+  /** The workspace's ordinary tab close, including its unsaved-changes guard. */
+  onCloseTab: (tabId: string) => Promise<void>;
+}
+
+interface WorkspaceSheetPresentationProps extends Omit<
+  WorkspaceSheetHostProps,
+  "workspaceKey" | "onCloseTab"
+> {
+  sheet: WorkspaceSheet;
+  onClose: () => Promise<void>;
+  onOpenAsTab: () => void;
+}
+
+/**
+ * Hosts the workspace's sheet tab over the compact tab strip. It renders in the workspace tree,
+ * not through a portal, so the panel keeps every context an ordinary pane has.
+ */
+export function WorkspaceSheetHost({ workspaceKey, onCloseTab, ...rest }: WorkspaceSheetHostProps) {
+  const sheet = useWorkspaceLayoutStore((state) =>
+    workspaceKey ? (state.sheetByWorkspace[workspaceKey] ?? null) : null,
+  );
+  const openSheetAsTab = useWorkspaceLayoutStore((state) => state.openSheetAsTab);
+  const sheetTabId = sheet?.tab.tabId ?? null;
+  const handleClose = useCallback(async () => {
+    if (sheetTabId) await onCloseTab(sheetTabId);
+  }, [onCloseTab, sheetTabId]);
+  const handleOpenAsTab = useCallback(() => {
+    if (workspaceKey) openSheetAsTab(workspaceKey);
+  }, [openSheetAsTab, workspaceKey]);
+  if (!sheet) return null;
+  return (
+    <WorkspaceSheetPresentation
+      {...rest}
+      sheet={sheet}
+      onClose={handleClose}
+      onOpenAsTab={handleOpenAsTab}
+    />
+  );
+}
+
+function WorkspaceSheetPresentation({
+  sheet,
+  serverId,
+  workspaceId,
+  isWorkspaceFocused,
+  buildContent,
+  onClose,
+  onOpenAsTab,
+}: WorkspaceSheetPresentationProps) {
+  const sheetRef = useRef<BottomSheet>(null);
+  const isScreenFocused = useIsFocused();
+  const ownsBackPress = isNative && isWorkspaceFocused && isScreenFocused;
+  const handleChange = useCallback(
+    (index: number) => {
+      if (index !== -1) return;
+      // A declined close (unsaved changes) leaves the tab in the sheet, so bring it back up.
+      void onClose().then(() => sheetRef.current?.snapToIndex(0));
+    },
+    [onClose],
+  );
+  useEffect(() => {
+    // Hidden workspaces stay mounted in the deck and under Settings; only the visible one owns back.
+    if (!ownsBackPress) return;
+    const subscription = BackHandler.addEventListener("hardwareBackPress", () => {
+      void onClose();
+      return true;
+    });
+    return () => subscription.remove();
+  }, [onClose, ownsBackPress]);
+
+  return (
+    <View style={styles.host} pointerEvents="box-none">
+      <ThemedBottomSheet
+        ref={sheetRef}
+        index={0}
+        snapPoints={SNAP_POINTS}
+        enableDynamicSizing={false}
+        enablePanDownToClose
+        enableContentPanningGesture={false}
+        backdropComponent={renderBackdrop}
+        onChange={handleChange}
+      >
+        <WorkspaceSheetBody
+          key={`${sheet.tab.tabId}:${sheet.tab.target.kind}`}
+          sheet={sheet}
+          serverId={serverId}
+          workspaceId={workspaceId}
+          isWorkspaceFocused={isWorkspaceFocused}
+          buildContent={buildContent}
+          onClose={onClose}
+          onOpenAsTab={onOpenAsTab}
+        />
+      </ThemedBottomSheet>
+    </View>
+  );
+}
+
+function WorkspaceSheetBody({
+  sheet,
+  serverId,
+  workspaceId,
+  isWorkspaceFocused,
+  buildContent,
+  onClose,
+  onOpenAsTab,
+}: WorkspaceSheetPresentationProps) {
+  const { t } = useTranslation();
+  const { tab } = sheet;
+  ensurePanelsRegistered();
+  const registration = getPanelRegistration(tab.target.kind);
+  if (!registration) {
+    throw new Error(`No panel registration for kind: ${tab.target.kind}`);
+  }
+  // The body is keyed by tab id and kind, so the registration and its descriptor hook stay fixed.
+  const descriptor = registration.useDescriptor(tab.target as never, {
+    serverId,
+    workspaceId,
+    tabId: tab.tabId,
+  });
+  const tabDescriptor = useMemo<WorkspaceTabDescriptor>(
+    () => ({
+      key: tab.tabId,
+      tabId: tab.tabId,
+      kind: tab.target.kind,
+      target: tab.target,
+      state: tab.state,
+    }),
+    [tab],
+  );
+  const content = useMemo(() => buildContent(tabDescriptor), [buildContent, tabDescriptor]);
+  const handleClose = useCallback(() => {
+    void onClose();
+  }, [onClose]);
+  // The sheet reaches the bottom of the screen, so it owns the system navigation inset.
+  const { bottom: bottomInset } = useSafeAreaInsets();
+  const contentStyle = useMemo(
+    () => [styles.content, { paddingBottom: bottomInset }],
+    [bottomInset],
+  );
+  const header = useMemo(
+    () => ({
+      title: descriptor.label,
+      leading: <ThemedPanelIcon icon={descriptor.icon} size={16} />,
+      actions: sheet.policy.openAsTab ? (
+        <Button size="sm" variant="secondary" onPress={onOpenAsTab}>
+          {t("workspace.sheet.openAsTab")}
+        </Button>
+      ) : undefined,
+    }),
+    [descriptor.icon, descriptor.label, onOpenAsTab, sheet.policy.openAsTab, t],
+  );
+
+  return (
+    <View style={styles.body} role="dialog" aria-label={descriptor.label}>
+      <SheetHeaderView header={header} onClose={handleClose} />
+      <View style={contentStyle}>
+        <WorkspacePaneContent
+          content={content}
+          isWorkspaceFocused={isWorkspaceFocused}
+          isPaneFocused={isWorkspaceFocused}
+        />
+      </View>
+    </View>
+  );
+}
+
+const styles = StyleSheet.create({
+  host: {
+    ...StyleSheet.absoluteFillObject,
+  },
+  body: {
+    flex: 1,
+    minHeight: 0,
+  },
+  content: {
+    flex: 1,
+    minHeight: 0,
+  },
+});

packages/app/src/stores/workspace-layout-store.test.ts

@@ -1,4 +1,4 @@
-import { beforeEach, describe, expect, it, vi } from "vitest";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 
 vi.mock("@react-native-async-storage/async-storage", () => {
   const storage = new Map<string, string>();
@@ -16,6 +16,9 @@ vi.mock("@react-native-async-storage/async-storage", () => {
 });
 
 import AsyncStorage from "@react-native-async-storage/async-storage";
+import type { DaemonClient } from "@getpaseo/client/internal/daemon-client";
+import { useSessionStore, type Agent } from "@/stores/session-store";
+import { setPanelInstanceAttributes } from "@/panels/panel-instance-attributes";
 import { createViewedTimelineSync } from "@/timeline/viewed-timeline-sync";
 import { buildWorkspaceTabPersistenceKey, type WorkspaceTab } from "@/workspace-tabs/model";
 import { defaultChangesState, type ChangesState } from "@/panels/changes/state";
@@ -24,6 +27,7 @@ import {
   canDismissPaneInLayout,
   collectAllPanes,
   collectAllTabs,
+  selectPaneFocusSuspended,
   createWorkspaceLayoutStore,
   observeOpenWorkspaceAgentIds,
   createDefaultLayout,
@@ -4438,3 +4442,580 @@ it("persists the once-only PR add after closing, and clears it when purging the
   expect(restored.getState().pullRequestTabAutoOpenedByWorkspace[workspaceKey]).toBeUndefined();
   expect(restored.getState().autoOpenPullRequestTab(workspaceKey, placement)).toBe("pull_request");
 });
+
+describe("compact workspace sheet", () => {
+  const workspaceKey = "server-1:sheet-ws";
+  const fileTarget = { kind: "file" as const, path: "src/index.ts" };
+
+  function createCompactStore() {
+    const store = createWorkspaceLayoutStore(workspaceLayoutIds);
+    store.setState({ layoutByWorkspace: {}, sheetByWorkspace: {} });
+    store.getState().setLayoutMode("compact");
+    store.getState().openTab({
+      workspaceKey,
+      target: { kind: "terminal", terminalId: "term-1" },
+      intent: "reveal",
+    });
+    return store;
+  }
+
+  function tabKinds(store: ReturnType<typeof createCompactStore>): string[] {
+    const layout = store.getState().layoutByWorkspace[workspaceKey];
+    const tabsById = new Map(collectAllTabs(layout.root).map((tab) => [tab.tabId, tab]));
+    return (findPaneById(layout.root, "main")?.tabIds ?? []).map(
+      (tabId) => tabsById.get(tabId)?.target.kind ?? "missing",
+    );
+  }
+
+  function seedAgents(agents: Array<Pick<Agent, "id" | "parentAgentId" | "workspaceId">>): void {
+    const timestamp = new Date("2026-04-21T10:00:00.000Z");
+    useSessionStore.getState().initializeSession("server-1", null as unknown as DaemonClient);
+    useSessionStore.getState().setAgents(
+      "server-1",
+      new Map(
+        agents.map((agent) => [
+          agent.id,
+          {
+            serverId: "server-1",
+            provider: "codex",
+            status: "idle",
+            turn: { phase: "idle", cancellationRequestId: null },
+            createdAt: timestamp,
+            updatedAt: timestamp,
+            lastUserMessageAt: null,
+            lastActivityAt: timestamp,
+            capabilities: {
+              supportsStreaming: true,
+              supportsSessionPersistence: true,
+              supportsDynamicModes: true,
+              supportsMcpServers: true,
+              supportsReasoningStream: true,
+              supportsToolInvocations: true,
+            },
+            currentModeId: null,
+            availableModes: [],
+            pendingPermissions: [],
+            persistence: null,
+            runtimeInfo: undefined,
+            lastUsage: undefined,
+            lastError: null,
+            title: agent.id,
+            cwd: "/repo",
+            model: null,
+            features: undefined,
+            thinkingOptionId: undefined,
+            requiresAttention: false,
+            attentionReason: null,
+            attentionTimestamp: null,
+            archivedAt: null,
+            labels: {},
+            projectPlacement: null,
+            ...agent,
+          } satisfies Agent,
+        ]),
+      ),
+    );
+  }
+
+  it("reveals a file in the sheet and leaves the tab strip unchanged", () => {
+    const store = createCompactStore();
+    const before = tabKinds(store);
+
+    const tabId = store.getState().openTab({ workspaceKey, target: fileTarget, intent: "reveal" });
+
+    expect(store.getState().sheetByWorkspace[workspaceKey]?.tab).toEqual({
+      tabId,
+      target: fileTarget,
+      createdAt: expect.any(Number),
+    });
+    expect(tabKinds(store)).toEqual(before);
+  });
+
+  it("focuses a target that is already open as a tab instead of using the sheet", () => {
+    const store = createCompactStore();
+    store.getState().setLayoutMode("split-panes");
+    const fileTabId = store
+      .getState()
+      .openTab({ workspaceKey, target: fileTarget, intent: "reveal" });
+    store.getState().openTab({
+      workspaceKey,
+      target: { kind: "terminal", terminalId: "term-1" },
+      intent: "reveal",
+    });
+    store.getState().setLayoutMode("compact");
+
+    const tabId = store.getState().openTab({ workspaceKey, target: fileTarget, intent: "reveal" });
+
+    expect(tabId).toBe(fileTabId);
+    expect(store.getState().sheetByWorkspace[workspaceKey]).toBeUndefined();
+    const layout = store.getState().layoutByWorkspace[workspaceKey];
+    expect(findPaneContainingTab(layout.root, fileTabId ?? "")?.focusedTabId).toBe(fileTabId);
+  });
+
+  it("replaces the sheet content with the next sheet target", () => {
+    const store = createCompactStore();
+    store.getState().openTab({ workspaceKey, target: fileTarget, intent: "reveal" });
+
+    const nextTarget = { kind: "file" as const, path: "src/other.ts" };
+    store.getState().openTab({ workspaceKey, target: nextTarget, intent: "reveal" });
+
+    expect(store.getState().sheetByWorkspace[workspaceKey]?.tab.target).toEqual(nextTarget);
+    expect(tabKinds(store)).toEqual(["terminal"]);
+  });
+
+  it("closes the sheet when a tab target is revealed, and keeps it for background opens", () => {
+    const store = createCompactStore();
+    store.getState().openTab({ workspaceKey, target: fileTarget, intent: "reveal" });
+
+    store.getState().openTab({
+      workspaceKey,
+      target: { kind: "file", path: "src/background.ts" },
+      intent: "background",
+    });
+    expect(store.getState().sheetByWorkspace[workspaceKey]?.tab.target).toEqual(fileTarget);
+    expect(tabKinds(store)).toEqual(["terminal", "file"]);
+
+    store.getState().openTab({
+      workspaceKey,
+      target: { kind: "terminal", terminalId: "term-2" },
+      intent: "reveal",
+    });
+    expect(store.getState().sheetByWorkspace[workspaceKey]).toBeUndefined();
+  });
+
+  it("moves the sheet tab into the focused pane with the same tab id and state", () => {
+    const store = createCompactStore();
+    const sheetTabId = store
+      .getState()
+      .openTab({ workspaceKey, target: fileTarget, intent: "reveal" });
+    if (!sheetTabId) throw new Error("Expected a sheet tab");
+    store.getState().setTabState(workspaceKey, sheetTabId, { scrollTop: 120 });
+
+    const tabId = store.getState().openSheetAsTab(workspaceKey);
+
+    expect(tabId).toBe(sheetTabId);
+    expect(store.getState().sheetByWorkspace[workspaceKey]).toBeUndefined();
+    const layout = store.getState().layoutByWorkspace[workspaceKey];
+    const pane = findPaneContainingTab(layout.root, sheetTabId);
+    expect(pane?.id).toBe(layout.focusedPaneId);
+    expect(pane?.focusedTabId).toBe(sheetTabId);
+    expect(collectAllTabs(layout.root).find((tab) => tab.tabId === sheetTabId)?.state).toEqual({
+      scrollTop: 120,
+    });
+  });
+
+  it("closes the sheet through the ordinary close and ignores focus requests for it", () => {
+    const store = createCompactStore();
+    const sheetTabId = store
+      .getState()
+      .openTab({ workspaceKey, target: fileTarget, intent: "reveal" });
+    if (!sheetTabId) throw new Error("Expected a sheet tab");
+    const layoutBefore = store.getState().layoutByWorkspace[workspaceKey];
+
+    store.getState().focusTab(workspaceKey, sheetTabId);
+    expect(store.getState().layoutByWorkspace[workspaceKey]).toBe(layoutBefore);
+
+    store.getState().closeTab(workspaceKey, sheetTabId);
+    expect(store.getState().sheetByWorkspace[workspaceKey]).toBeUndefined();
+    expect(tabKinds(store)).toEqual(["terminal"]);
+  });
+
+  it("suspends pane focus while the sheet covers the panes", () => {
+    const store = createCompactStore();
+    expect(selectPaneFocusSuspended(store.getState(), workspaceKey)).toBe(false);
+
+    const sheetTabId = store
+      .getState()
+      .openTab({ workspaceKey, target: fileTarget, intent: "reveal" });
+    expect(selectPaneFocusSuspended(store.getState(), workspaceKey)).toBe(true);
+
+    store.getState().closeTab(workspaceKey, sheetTabId ?? "");
+    expect(selectPaneFocusSuspended(store.getState(), workspaceKey)).toBe(false);
+  });
+
+  it("never uses the sheet on wide layouts", () => {
+    const store = createCompactStore();
+    store.getState().setLayoutMode("split-panes");
+
+    store.getState().openTab({ workspaceKey, target: fileTarget, intent: "reveal" });
+
+    expect(store.getState().sheetByWorkspace).toEqual({});
+    expect(tabKinds(store)).toEqual(["terminal", "file"]);
+  });
+
+  it("keeps the sheet content as a tab when the layout widens", () => {
+    const store = createCompactStore();
+    const sheetTabId = store
+      .getState()
+      .openTab({ workspaceKey, target: fileTarget, intent: "reveal" });
+
+    store.getState().setLayoutMode("split-panes");
+
+    expect(store.getState().sheetByWorkspace).toEqual({});
+    const layout = store.getState().layoutByWorkspace[workspaceKey];
+    expect(findPaneContainingTab(layout.root, sheetTabId ?? "")?.focusedTabId).toBe(sheetTabId);
+  });
+
+  it("opens subagents and provider subagents in the sheet but keeps root agents as tabs", () => {
+    seedAgents([
+      { id: "root", parentAgentId: null, workspaceId: "sheet-ws" },
+      { id: "child", parentAgentId: "root", workspaceId: "sheet-ws" },
+      { id: "other-workspace-child", parentAgentId: "root", workspaceId: "other-ws" },
+    ]);
+    const store = createCompactStore();
+
+    store.getState().openTab({
+      workspaceKey,
+      target: { kind: "agent", agentId: "child" },
+      intent: "reveal",
+    });
+    expect(store.getState().sheetByWorkspace[workspaceKey]).toMatchObject({
+      tab: { target: { kind: "agent", agentId: "child" } },
+      policy: { openAsTab: true },
+    });
+
+    store.getState().openTab({
+      workspaceKey,
+      target: { kind: "provider_subagent", parentAgentId: "root", subagentId: "task-1" },
+      intent: "reveal",
+    });
+    expect(store.getState().sheetByWorkspace[workspaceKey]).toMatchObject({
+      tab: { target: { kind: "provider_subagent", subagentId: "task-1" } },
+      policy: { openAsTab: false },
+    });
+
+    for (const agentId of ["root", "other-workspace-child"]) {
+      store
+        .getState()
+        .openTab({ workspaceKey, target: { kind: "agent", agentId }, intent: "reveal" });
+    }
+    expect(store.getState().sheetByWorkspace[workspaceKey]).toBeUndefined();
+    expect(tabKinds(store)).toEqual(["terminal", "agent", "agent"]);
+  });
+
+  it("gives a sheet target opened from inside the sheet the sheet's own parent", () => {
+    seedAgents([
+      { id: "root", parentAgentId: null, workspaceId: "sheet-ws" },
+      { id: "child", parentAgentId: "root", workspaceId: "sheet-ws" },
+    ]);
+    const store = createCompactStore();
+    const rootTabId = store
+      .getState()
+      .openTab({ workspaceKey, target: { kind: "agent", agentId: "root" }, intent: "reveal" });
+    const childSheetTabId = store.getState().openTab({
+      workspaceKey,
+      target: { kind: "agent", agentId: "child" },
+      intent: "reveal",
+      parentTabId: rootTabId ?? undefined,
+    });
+
+    const fileSheetTabId = store.getState().openTab({
+      workspaceKey,
+      target: fileTarget,
+      intent: "reveal",
+      parentTabId: childSheetTabId ?? undefined,
+    });
+    expect(store.getState().sheetByWorkspace[workspaceKey]?.parentTabId).toBe(rootTabId);
+
+    store.getState().openSheetAsTab(workspaceKey);
+    const layout = store.getState().layoutByWorkspace[workspaceKey];
+    expect(layout.parentTabIdByTabId?.[fileSheetTabId ?? ""]).toBe(rootTabId);
+  });
+
+  it("closes the sheet and reveals the tab when a tab target is opened from inside the sheet", () => {
+    seedAgents([
+      { id: "root", parentAgentId: null, workspaceId: "sheet-ws" },
+      { id: "child", parentAgentId: "root", workspaceId: "sheet-ws" },
+      { id: "second-root", parentAgentId: null, workspaceId: "sheet-ws" },
+    ]);
+    const store = createCompactStore();
+    const rootTabId = store
+      .getState()
+      .openTab({ workspaceKey, target: { kind: "agent", agentId: "root" }, intent: "reveal" });
+    const openChildSheet = () =>
+      store.getState().openTab({
+        workspaceKey,
+        target: { kind: "agent", agentId: "child" },
+        intent: "reveal",
+        parentTabId: rootTabId ?? undefined,
+      }) ?? undefined;
+
+    const terminalTabId = store.getState().openTab({
+      workspaceKey,
+      target: { kind: "terminal", terminalId: "term-1" },
+      intent: "reveal",
+      parentTabId: openChildSheet(),
+    });
+    expect(store.getState().sheetByWorkspace[workspaceKey]).toBeUndefined();
+    let layout = store.getState().layoutByWorkspace[workspaceKey];
+    expect(findPaneById(layout.root, "main")?.focusedTabId).toBe(terminalTabId);
+
+    const secondRootTabId = store.getState().openTab({
+      workspaceKey,
+      target: { kind: "agent", agentId: "second-root" },
+      intent: "reveal",
+      parentTabId: openChildSheet(),
+    });
+    expect(store.getState().sheetByWorkspace[workspaceKey]).toBeUndefined();
+    layout = store.getState().layoutByWorkspace[workspaceKey];
+    expect(findPaneById(layout.root, "main")?.focusedTabId).toBe(secondRootTabId);
+    expect(layout.parentTabIdByTabId?.[secondRootTabId ?? ""]).toBe(rootTabId);
+    expect(tabKinds(store)).toEqual(["terminal", "agent", "agent"]);
+  });
+
+  describe("unsaved sheet work", () => {
+    const sheetPanel = (tabId: string) => ({
+      serverId: "server-1",
+      workspaceId: "sheet-ws",
+      tabId,
+    });
+    let modifiedTabId: string | null = null;
+
+    function openModifiedFileSheet(store: ReturnType<typeof createCompactStore>): string {
+      const tabId = store
+        .getState()
+        .openTab({ workspaceKey, target: fileTarget, intent: "reveal" });
+      if (!tabId) throw new Error("Expected a sheet tab");
+      store.getState().setTabState(workspaceKey, tabId, { scrollTop: 40 });
+      setPanelInstanceAttributes(sheetPanel(tabId), { modified: true });
+      modifiedTabId = tabId;
+      return tabId;
+    }
+
+    function stripTab(store: ReturnType<typeof createCompactStore>, tabId: string) {
+      const layout = store.getState().layoutByWorkspace[workspaceKey];
+      return collectAllTabs(layout.root).find((tab) => tab.tabId === tabId) ?? null;
+    }
+
+    function focusedTabId(store: ReturnType<typeof createCompactStore>): string | null {
+      const layout = store.getState().layoutByWorkspace[workspaceKey];
+      return findPaneById(layout.root, layout.focusedPaneId)?.focusedTabId ?? null;
+    }
+
+    afterEach(() => {
+      if (modifiedTabId) setPanelInstanceAttributes(sheetPanel(modifiedTabId), { modified: false });
+      modifiedTabId = null;
+    });
+
+    function sheetTabId(store: ReturnType<typeof createCompactStore>): string | null {
+      return store.getState().sheetByWorkspace[workspaceKey]?.tab.tabId ?? null;
+    }
+
+    it("opens the next sheet target as a tab under a modified sheet", () => {
+      const store = createCompactStore();
+      const modifiedSheetTabId = openModifiedFileSheet(store);
+
+      const nextTarget = { kind: "file" as const, path: "src/other.ts" };
+      const nextTabId = store
+        .getState()
+        .openTab({ workspaceKey, target: nextTarget, intent: "reveal" });
+
+      expect(sheetTabId(store)).toBe(modifiedSheetTabId);
+      expect(store.getState().sheetByWorkspace[workspaceKey]?.tab.state).toEqual({ scrollTop: 40 });
+      expect(stripTab(store, nextTabId ?? "")).toMatchObject({ target: nextTarget });
+      expect(focusedTabId(store)).toBe(nextTabId);
+    });
+
+    it("opens a tab target under a modified sheet without closing it", () => {
+      const store = createCompactStore();
+      const modifiedSheetTabId = openModifiedFileSheet(store);
+
+      const terminalTabId = store.getState().openTab({
+        workspaceKey,
+        target: { kind: "terminal", terminalId: "term-2" },
+        intent: "reveal",
+      });
+
+      expect(sheetTabId(store)).toBe(modifiedSheetTabId);
+      expect(focusedTabId(store)).toBe(terminalTabId);
+    });
+
+    it("keeps a modified sheet open when another tab is chosen", () => {
+      const store = createCompactStore();
+      const secondTabId = store.getState().openTab({
+        workspaceKey,
+        target: { kind: "terminal", terminalId: "term-2" },
+        intent: "reveal",
+      });
+      const firstTabId = store.getState().getWorkspaceTabs(workspaceKey)[0]?.tabId ?? "";
+      const modifiedSheetTabId = openModifiedFileSheet(store);
+
+      store.getState().focusTab(workspaceKey, firstTabId);
+      expect(sheetTabId(store)).toBe(modifiedSheetTabId);
+      expect(focusedTabId(store)).toBe(firstTabId);
+
+      store.getState().selectTabInPane(workspaceKey, "main", secondTabId ?? "");
+      expect(sheetTabId(store)).toBe(modifiedSheetTabId);
+      expect(focusedTabId(store)).toBe(secondTabId);
+
+      store.getState().closeTab(workspaceKey, modifiedSheetTabId);
+      expect(sheetTabId(store)).toBeNull();
+      expect(tabKinds(store)).toEqual(["terminal", "terminal"]);
+    });
+
+    it("replaces an unmodified sheet with the next sheet target", () => {
+      const store = createCompactStore();
+      store.getState().openTab({ workspaceKey, target: fileTarget, intent: "reveal" });
+
+      store.getState().openTab({
+        workspaceKey,
+        target: { kind: "file", path: "src/other.ts" },
+        intent: "reveal",
+      });
+
+      expect(store.getState().sheetByWorkspace[workspaceKey]?.tab.target).toEqual({
+        kind: "file",
+        path: "src/other.ts",
+      });
+      expect(tabKinds(store)).toEqual(["terminal"]);
+    });
+
+    it("closes an unmodified sheet when another tab is chosen", () => {
+      const store = createCompactStore();
+      const secondTabId = store.getState().openTab({
+        workspaceKey,
+        target: { kind: "terminal", terminalId: "term-2" },
+        intent: "reveal",
+      });
+      const firstTabId = store.getState().getWorkspaceTabs(workspaceKey)[0]?.tabId ?? "";
+
+      store.getState().openTab({ workspaceKey, target: fileTarget, intent: "reveal" });
+      store.getState().focusTab(workspaceKey, firstTabId);
+      expect(sheetTabId(store)).toBeNull();
+      expect(focusedTabId(store)).toBe(firstTabId);
+
+      store.getState().openTab({ workspaceKey, target: fileTarget, intent: "reveal" });
+      store.getState().selectTabInPane(workspaceKey, "main", secondTabId ?? "");
+      expect(sheetTabId(store)).toBeNull();
+      expect(focusedTabId(store)).toBe(secondTabId);
+      expect(tabKinds(store)).toEqual(["terminal", "terminal"]);
+    });
+  });
+
+  it("closes the sheet when its agent leaves the workspace", () => {
+    seedAgents([
+      { id: "root", parentAgentId: null, workspaceId: "sheet-ws" },
+      { id: "child", parentAgentId: "root", workspaceId: "sheet-ws" },
+    ]);
+    const store = createCompactStore();
+    store.getState().openTab({
+      workspaceKey,
+      target: { kind: "agent", agentId: "child" },
+      intent: "reveal",
+    });
+    const snapshot = {
+      terminalsHydrated: false,
+      activeAgentIds: ["root"],
+      autoOpenAgentIds: [],
+      knownTerminalIds: ["term-1"],
+      standaloneTerminalIds: ["term-1"],
+      hasActivePendingTerminalCreate: false,
+    };
+
+    store.getState().reconcileTabs(workspaceKey, { ...snapshot, agentsHydrated: false });
+    expect(store.getState().sheetByWorkspace[workspaceKey]).toBeDefined();
+
+    store.getState().reconcileTabs(workspaceKey, { ...snapshot, agentsHydrated: true });
+    expect(store.getState().sheetByWorkspace[workspaceKey]).toBeUndefined();
+  });
+
+  it("closes the sheet when its agent stops being a subagent, leaving one tab for it", () => {
+    seedAgents([
+      { id: "root", parentAgentId: null, workspaceId: "sheet-ws" },
+      { id: "child", parentAgentId: "root", workspaceId: "sheet-ws" },
+    ]);
+    const store = createCompactStore();
+    const sheetTabId = store.getState().openTab({
+      workspaceKey,
+      target: { kind: "agent", agentId: "child" },
+      intent: "reveal",
+    });
+    const snapshot = {
+      agentsHydrated: true,
+      terminalsHydrated: false,
+      activeAgentIds: ["root", "child"],
+      autoOpenAgentIds: ["child"],
+      knownTerminalIds: ["term-1"],
+      standaloneTerminalIds: ["term-1"],
+      hasActivePendingTerminalCreate: false,
+    };
+
+    store.getState().reconcileTabs(workspaceKey, { ...snapshot, autoOpenAgentIds: [] });
+    expect(store.getState().sheetByWorkspace[workspaceKey]?.tab.tabId).toBe(sheetTabId);
+
+    seedAgents([
+      { id: "root", parentAgentId: null, workspaceId: "sheet-ws" },
+      { id: "child", parentAgentId: null, workspaceId: "sheet-ws" },
+    ]);
+    store.getState().reconcileTabs(workspaceKey, snapshot);
+
+    expect(store.getState().sheetByWorkspace[workspaceKey]).toBeUndefined();
+    const layout = store.getState().layoutByWorkspace[workspaceKey];
+    expect(
+      collectAllTabs(layout.root).filter(
+        (tab) => tab.target.kind === "agent" && tab.target.agentId === "child",
+      ),
+    ).toHaveLength(1);
+  });
+
+  it("re-resolves the sheet policy when the sheet tab gets another target", () => {
+    seedAgents([
+      { id: "root", parentAgentId: null, workspaceId: "sheet-ws" },
+      { id: "child", parentAgentId: "root", workspaceId: "sheet-ws" },
+    ]);
+    const store = createCompactStore();
+    const sheetTabId = store.getState().openTab({
+      workspaceKey,
+      target: { kind: "provider_subagent", parentAgentId: "root", subagentId: "task-1" },
+      intent: "reveal",
+    });
+
+    const replacedTabId = store
+      .getState()
+      .replaceTab(workspaceKey, sheetTabId ?? "", { kind: "agent", agentId: "child" });
+
+    expect(replacedTabId).toBe(sheetTabId);
+    expect(store.getState().sheetByWorkspace[workspaceKey]).toMatchObject({
+      tab: { tabId: sheetTabId, target: { kind: "agent", agentId: "child" } },
+      policy: { openAsTab: true },
+    });
+  });
+
+  it("moves the sheet tab into the tab strip when it gets a target the sheet does not host", () => {
+    const store = createCompactStore();
+    const sheetTabId = store
+      .getState()
+      .openTab({ workspaceKey, target: fileTarget, intent: "reveal" });
+
+    const replacedTabId = store.getState().replaceTab(workspaceKey, sheetTabId ?? "", {
+      kind: "terminal",
+      terminalId: "term-2",
+    });
+
+    expect(store.getState().sheetByWorkspace[workspaceKey]).toBeUndefined();
+    expect(replacedTabId).toBe(sheetTabId);
+    const layout = store.getState().layoutByWorkspace[workspaceKey];
+    expect(findPaneById(layout.root, "main")?.focusedTabId).toBe(sheetTabId);
+    expect(tabKinds(store)).toEqual(["terminal", "terminal"]);
+  });
+
+  it("keeps a chat shown in the sheet open for timeline sync until the sheet closes", () => {
+    const store = createCompactStore();
+    seedAgents([
+      { id: "root", parentAgentId: null, workspaceId: "sheet-ws" },
+      { id: "child", parentAgentId: "root", workspaceId: "sheet-ws" },
+    ]);
+    const received: string[][] = [];
+    const stop = observeOpenWorkspaceAgentIds("server-1", (ids) => received.push(ids), store);
+
+    store.getState().openTab({
+      workspaceKey,
+      target: { kind: "agent", agentId: "child" },
+      intent: "reveal",
+    });
+    store.getState().closeSheet(workspaceKey);
+    stop();
+
+    expect(received).toEqual([[], ["child"], []]);
+  });
+});

packages/app/src/stores/workspace-layout-store.ts

@@ -59,7 +59,12 @@ import {
   type WorkspaceTabSnapshot,
   type WorkspaceLayout,
 } from "@/stores/workspace-layout-actions";
-import { normalizeWorkspaceTabTarget } from "@/workspace-tabs/identity";
+import {
+  buildDeterministicWorkspaceTabId,
+  normalizeWorkspaceTabTarget,
+} from "@/workspace-tabs/identity";
+import { isSheetTabModified, resolveCompactSheetPolicy } from "@/workspace-tabs/compact-sheet";
+import { panelResourceKey, type CompactSheetPolicy } from "@/panels/panel-manifest";
 import { createValidatedPersistStorage } from "@/storage/validated-persist-storage";
 import { panelTargetSupportsHostForWorkspaceKey } from "@/plugins/workspace-panels/locations";
 
@@ -105,7 +110,24 @@ export interface OpenWorkspaceTabInput {
   state?: JsonValue;
 }
 
+/**
+ * How the app shell lays out a workspace: compact (one tab at a time, plus the sheet), a single
+ * wide pane (touch tablets, which cannot render splits), or split panes.
+ */
+export type WorkspaceLayoutMode = "compact" | "single-pane" | "split-panes";
+
+/** A tab hosted by the compact workspace sheet instead of a pane. */
+export interface WorkspaceSheet {
+  tab: WorkspaceTab;
+  policy: CompactSheetPolicy;
+  parentTabId: string | null;
+}
+
 interface WorkspaceLayoutStore {
+  /** Published once by the app shell; callers never pass a form factor to open a tab. */
+  layoutMode: WorkspaceLayoutMode;
+  /** At most one sheet tab per workspace. Not persisted: a reload returns to the tab strip. */
+  sheetByWorkspace: Record<string, WorkspaceSheet>;
   layoutByWorkspace: Record<string, WorkspaceLayout>;
   splitSizesByWorkspace: Record<string, Record<string, number[]>>;
   explorerSidebarWidthByWorkspace: Record<string, number>;
@@ -116,7 +138,19 @@ interface WorkspaceLayoutStore {
   sidePaneIdByWorkspace: Record<string, string | null>;
   /** Workspaces where PR detection already added its tab once; a closed tab never returns. */
   pullRequestTabAutoOpenedByWorkspace: Record<string, true>;
+  /**
+   * The one entry point for showing a target. On compact layouts a target whose panel declares
+   * `compactSheet` opens in the workspace sheet; the returned tab id is the sheet tab's id.
+   */
   openTab: (input: OpenWorkspaceTabInput) => string | null;
+  /** Whether `openTab` with this input would show the target in the sheet. */
+  opensInSheet: (
+    input: Pick<OpenWorkspaceTabInput, "workspaceKey" | "target" | "intent">,
+  ) => boolean;
+  setLayoutMode: (mode: WorkspaceLayoutMode) => void;
+  closeSheet: (workspaceKey: string) => void;
+  /** Moves the sheet's tab, keeping its tab id, into the focused pane. */
+  openSheetAsTab: (workspaceKey: string) => string | null;
   /** Placement resolves lazily so an already acknowledged workspace never creates a side pane. */
   autoOpenPullRequestTab: (
     workspaceKey: string,
@@ -472,6 +506,17 @@ export function selectExplorerSidebarPaneId(
   return resolveExplorerSidebarPaneId(layout, state.explorerSidebarPaneIdByWorkspace[workspaceKey]);
 }
 
+/** A workspace's panes give up keyboard focus while focus is parked or the sheet covers them. */
+export function selectPaneFocusSuspended(
+  state: Pick<WorkspaceLayoutStore, "focusRestorationByWorkspace" | "sheetByWorkspace">,
+  workspaceKey: string,
+): boolean {
+  return (
+    Boolean(state.focusRestorationByWorkspace[workspaceKey]?.restorePaneId) ||
+    workspaceKey in state.sheetByWorkspace
+  );
+}
+
 /** Whether the Explorer sidebar pane is currently on screen. */
 export function selectIsExplorerSidebarVisible(
   state: ExplorerSidebarState,
@@ -637,6 +682,189 @@ function attachParentTab(input: {
   });
 }
 
+function withoutSheet(
+  state: Pick<WorkspaceLayoutStore, "sheetByWorkspace">,
+  workspaceKey: string,
+): Pick<WorkspaceLayoutStore, "sheetByWorkspace"> {
+  if (!(workspaceKey in state.sheetByWorkspace)) {
+    return { sheetByWorkspace: state.sheetByWorkspace };
+  }
+  const { [workspaceKey]: _removed, ...sheetByWorkspace } = state.sheetByWorkspace;
+  return { sheetByWorkspace };
+}
+
+function isTargetOpenAsTab(layout: WorkspaceLayout | undefined, target: WorkspaceTabTarget) {
+  if (!layout) return false;
+  const resourceKey = panelResourceKey(target);
+  return collectAllTabs(layout.root).some((tab) => panelResourceKey(tab.target) === resourceKey);
+}
+
+function resolveSheetPolicy(
+  state: Pick<WorkspaceLayoutStore, "layoutMode" | "layoutByWorkspace">,
+  workspaceKey: string,
+  target: WorkspaceTabTarget,
+  intent: WorkspaceTabOpenIntent,
+): CompactSheetPolicy | null {
+  if (state.layoutMode !== "compact" || intent === "background") return null;
+  if (intent === "reveal" && isTargetOpenAsTab(state.layoutByWorkspace[workspaceKey], target)) {
+    return null;
+  }
+  return resolveCompactSheetPolicy(workspaceKey, target);
+}
+
+/**
+ * The parent a newly opened tab links to. A target opened from inside the sheet names the sheet's
+ * tab, which the open replaces or closes, so it inherits the sheet's own parent instead.
+ */
+function resolveOpenParentTabId(
+  state: Pick<WorkspaceLayoutStore, "sheetByWorkspace">,
+  workspaceKey: string,
+  parentTabId: string | undefined,
+): string | null {
+  const sheet = state.sheetByWorkspace[workspaceKey];
+  if (sheet && parentTabId === sheet.tab.tabId) return sheet.parentTabId;
+  return parentTabId ?? null;
+}
+
+/** A reveal of the resource the sheet already shows keeps the sheet's tab. */
+function sheetKeepsTabFor(
+  sheet: WorkspaceSheet | undefined,
+  target: WorkspaceTabTarget,
+  intent: WorkspaceTabOpenIntent,
+): sheet is WorkspaceSheet {
+  return (
+    intent === "reveal" &&
+    sheet !== undefined &&
+    panelResourceKey(sheet.tab.target) === panelResourceKey(target)
+  );
+}
+
+function placeTargetInSheet(input: {
+  /** The sheet whose tab the target keeps, if any. */
+  kept: WorkspaceSheet | undefined;
+  target: WorkspaceTabTarget;
+  intent: WorkspaceTabOpenIntent;
+  policy: CompactSheetPolicy;
+  parentTabId: string | null;
+  state: JsonValue | undefined;
+}): WorkspaceSheet {
+  if (input.kept) {
+    return { ...input.kept, tab: { ...input.kept.tab, target: input.target } };
+  }
+  return {
+    tab: {
+      tabId:
+        input.intent === "reveal"
+          ? buildDeterministicWorkspaceTabId(input.target)
+          : createWorkspaceTabInstanceId(),
+      target: input.target,
+      createdAt: Date.now(),
+      ...(input.state !== undefined ? { state: input.state } : {}),
+    },
+    policy: input.policy,
+    parentTabId: input.parentTabId,
+  };
+}
+
+/** Moves a workspace's sheet tab into its focused pane, keeping its tab id and state. */
+function promoteSheetToTab(
+  state: WorkspaceLayoutStore,
+  workspaceKey: string,
+): { tabId: string; patch: Partial<WorkspaceLayoutStore> } | null {
+  const sheet = state.sheetByWorkspace[workspaceKey];
+  if (!sheet) return null;
+  const { tab } = sheet;
+  const placement = getOpenTabPlacement(state, workspaceKey, tab.target, FOCUSED_PANE_PLACEMENT);
+  const input = {
+    ...placement,
+    target: tab.target,
+    now: tab.createdAt,
+    createTabId: () => tab.tabId,
+    state: tab.state,
+  };
+  const result = isTargetOpenAsTab(placement.layout, tab.target)
+    ? revealTargetInLayout(input)
+    : createTabInLayout(input);
+  if (!result) return null;
+  const nextLayout = attachParentTab({
+    layout: keepWorkspaceFocusOutOfExplorerSidebar(
+      result.layout,
+      placement.explorerSidebarPaneId,
+      placement.layout.focusedPaneId,
+    ),
+    childTabId: result.tabId,
+    parentTabId: sheet.parentTabId,
+  });
+  return {
+    tabId: result.tabId,
+    patch: {
+      ...withoutSheet(state, workspaceKey),
+      ...withoutFocusRestoration(state, workspaceKey),
+      hiddenAgentIdsByWorkspace:
+        tab.target.kind === "agent"
+          ? removeAgentIdFromWorkspaceSet(
+              state.hiddenAgentIdsByWorkspace,
+              workspaceKey,
+              tab.target.agentId,
+            )
+          : state.hiddenAgentIdsByWorkspace,
+      layoutByWorkspace: { ...state.layoutByWorkspace, [workspaceKey]: nextLayout },
+    },
+  };
+}
+
+/**
+ * A sheet holding unsaved work is never implicitly displaced: moving its tab would remount the
+ * panel and drop the work. Only the explicit, guarded close removes it.
+ */
+function sheetHoldsUnsavedWork(
+  state: Pick<WorkspaceLayoutStore, "sheetByWorkspace">,
+  workspaceKey: string,
+): boolean {
+  const sheet = state.sheetByWorkspace[workspaceKey];
+  return sheet !== undefined && isSheetTabModified(workspaceKey, sheet.tab.tabId);
+}
+
+/**
+ * Where an open lands in the sheet, or null when it opens as a tab. A reveal of the resource the
+ * sheet shows keeps its tab; any other target never displaces unsaved work and opens as a tab.
+ */
+function resolveSheetPlacement(
+  state: WorkspaceLayoutStore,
+  workspaceKey: string,
+  target: WorkspaceTabTarget,
+  intent: WorkspaceTabOpenIntent,
+): { policy: CompactSheetPolicy; kept: WorkspaceSheet | undefined } | null {
+  const policy = resolveSheetPolicy(state, workspaceKey, target, intent);
+  if (!policy) return null;
+  const sheet = state.sheetByWorkspace[workspaceKey];
+  if (sheetKeepsTabFor(sheet, target, intent)) return { policy, kept: sheet };
+  return sheetHoldsUnsavedWork(state, workspaceKey) ? null : { policy, kept: undefined };
+}
+
+/** Closes the sheet because navigation moved on underneath it, unless it holds unsaved work. */
+function dismissSheet(
+  state: WorkspaceLayoutStore,
+  workspaceKey: string,
+): Partial<WorkspaceLayoutStore> {
+  return sheetHoldsUnsavedWork(state, workspaceKey) ? {} : withoutSheet(state, workspaceKey);
+}
+
+/**
+ * A sheet agent closes once the workspace no longer has it (archived or gone), like its tab would,
+ * and once it stops being a subagent: as a root agent, reconcile gives it its own tab, and the sheet
+ * would hold a second tab with the same id.
+ */
+function sheetAgentNoLongerFits(
+  workspaceKey: string,
+  sheet: WorkspaceSheet | undefined,
+  snapshot: WorkspaceTabSnapshot,
+): boolean {
+  if (sheet?.tab.target.kind !== "agent" || !snapshot.agentsHydrated) return false;
+  if (!new Set(snapshot.activeAgentIds).has(sheet.tab.target.agentId)) return true;
+  return resolveCompactSheetPolicy(workspaceKey, sheet.tab.target) === null;
+}
+
 /**
  * Splits an Explorer sidebar out of the pane the user is in. Only reached by layouts saved
  * before the Explorer became part of the default tree; new ones are born with it.
@@ -676,6 +904,8 @@ export function createWorkspaceLayoutStore(
           }
           return tabId;
         },
+        layoutMode: "split-panes",
+        sheetByWorkspace: {},
         layoutByWorkspace: {},
         splitSizesByWorkspace: {},
         explorerSidebarWidthByWorkspace: {},
@@ -691,6 +921,31 @@ export function createWorkspaceLayoutStore(
           if (!normalizedWorkspaceKey || !normalizedTarget) {
             return null;
           }
+          const parentTabId = resolveOpenParentTabId(
+            get(),
+            normalizedWorkspaceKey,
+            input.parentTabId,
+          );
+          const sheetPlacement = resolveSheetPlacement(
+            get(),
+            normalizedWorkspaceKey,
+            normalizedTarget,
+            input.intent,
+          );
+          if (sheetPlacement) {
+            const sheet = placeTargetInSheet({
+              kept: sheetPlacement.kept,
+              target: normalizedTarget,
+              intent: input.intent,
+              policy: sheetPlacement.policy,
+              parentTabId,
+              state: input.state,
+            });
+            set((state) => ({
+              sheetByWorkspace: { ...state.sheetByWorkspace, [normalizedWorkspaceKey]: sheet },
+            }));
+            return sheet.tab.tabId;
+          }
           const placement = getOpenTabPlacement(
             get(),
             normalizedWorkspaceKey,
@@ -734,6 +989,8 @@ export function createWorkspaceLayoutStore(
           const shouldPinAgent = input.pin === true && normalizedTarget.kind === "agent";
           set((state) => ({
             ...withoutFocusRestoration(state, normalizedWorkspaceKey),
+            // A tab brought to the front dismisses the sheet over it.
+            ...(input.intent === "background" ? {} : dismissSheet(state, normalizedWorkspaceKey)),
             hiddenAgentIdsByWorkspace:
               normalizedTarget.kind !== "agent"
                 ? state.hiddenAgentIdsByWorkspace
@@ -751,17 +1008,52 @@ export function createWorkspaceLayoutStore(
               : state.pinnedAgentIdsByWorkspace,
             layoutByWorkspace: {
               ...state.layoutByWorkspace,
-              [normalizedWorkspaceKey]: input.parentTabId
+              [normalizedWorkspaceKey]: parentTabId
                 ? attachParentTab({
                     layout: nextLayout,
                     childTabId: result.tabId,
-                    parentTabId: input.parentTabId,
+                    parentTabId,
                   })
                 : nextLayout,
             },
           }));
           return result.tabId;
         },
+        opensInSheet: (input) => {
+          const normalizedWorkspaceKey = trimNonEmpty(input.workspaceKey);
+          const normalizedTarget = normalizeWorkspaceTabTarget(input.target);
+          if (!normalizedWorkspaceKey || !normalizedTarget) return false;
+          return (
+            resolveSheetPlacement(get(), normalizedWorkspaceKey, normalizedTarget, input.intent) !==
+            null
+          );
+        },
+        setLayoutMode: (mode) => {
+          set((state) => {
+            if (state.layoutMode === mode) return state;
+            if (mode === "compact") return { layoutMode: mode };
+            // Widening keeps whatever the user had open in the sheet as an ordinary tab.
+            const next: WorkspaceLayoutStore = { ...state, layoutMode: mode };
+            for (const workspaceKey of Object.keys(state.sheetByWorkspace)) {
+              const promoted = promoteSheetToTab(next, workspaceKey);
+              Object.assign(next, promoted?.patch ?? withoutSheet(next, workspaceKey));
+            }
+            return next;
+          });
+        },
+        closeSheet: (workspaceKey) => {
+          const normalizedWorkspaceKey = trimNonEmpty(workspaceKey);
+          if (!normalizedWorkspaceKey) return;
+          set((state) => withoutSheet(state, normalizedWorkspaceKey));
+        },
+        openSheetAsTab: (workspaceKey) => {
+          const normalizedWorkspaceKey = trimNonEmpty(workspaceKey);
+          if (!normalizedWorkspaceKey) return null;
+          const promoted = promoteSheetToTab(get(), normalizedWorkspaceKey);
+          if (!promoted) return null;
+          set(promoted.patch);
+          return promoted.tabId;
+        },
         showExplorerSidebar: (workspaceKey) => {
           const normalizedWorkspaceKey = trimNonEmpty(workspaceKey);
           if (!normalizedWorkspaceKey) {
@@ -881,6 +1173,9 @@ export function createWorkspaceLayoutStore(
           }
 
           set((state) => {
+            if (state.sheetByWorkspace[normalizedWorkspaceKey]?.tab.tabId === normalizedTabId) {
+              return withoutSheet(state, normalizedWorkspaceKey);
+            }
             const layout = getWorkspaceLayout(state.layoutByWorkspace, normalizedWorkspaceKey);
             const explorerSidebarPaneId = resolveExplorerSidebarPaneId(
               layout,
@@ -988,6 +1283,8 @@ export function createWorkspaceLayoutStore(
 
             return {
               ...withoutFocusRestoration(state, normalizedWorkspaceKey),
+              // Choosing a tab dismisses the sheet that covers it.
+              ...dismissSheet(state, normalizedWorkspaceKey),
               layoutByWorkspace: {
                 ...state.layoutByWorkspace,
                 [normalizedWorkspaceKey]: nextLayout,
@@ -1011,6 +1308,8 @@ export function createWorkspaceLayoutStore(
             });
             return nextLayout
               ? {
+                  // Choosing a tab dismisses the sheet that covers it.
+                  ...dismissSheet(state, normalizedWorkspaceKey),
                   layoutByWorkspace: {
                     ...state.layoutByWorkspace,
                     [normalizedWorkspaceKey]: nextLayout,
@@ -1024,6 +1323,31 @@ export function createWorkspaceLayoutStore(
           const normalizedTabId = trimNonEmpty(tabId);
           const normalizedTarget = normalizeWorkspaceTabTarget(target);
           if (!normalizedWorkspaceKey || !normalizedTabId || !normalizedTarget) return null;
+          const sheet = get().sheetByWorkspace[normalizedWorkspaceKey];
+          if (sheet?.tab.tabId === normalizedTabId) {
+            const policy = resolveCompactSheetPolicy(normalizedWorkspaceKey, normalizedTarget);
+            const replaced: WorkspaceSheet = {
+              ...sheet,
+              policy: policy ?? sheet.policy,
+              tab: {
+                ...sheet.tab,
+                target: normalizedTarget,
+                ...(tabState !== undefined ? { state: tabState } : {}),
+              },
+            };
+            const next: WorkspaceLayoutStore = {
+              ...get(),
+              sheetByWorkspace: { ...get().sheetByWorkspace, [normalizedWorkspaceKey]: replaced },
+            };
+            if (policy) {
+              set({ sheetByWorkspace: next.sheetByWorkspace });
+              return normalizedTabId;
+            }
+            // A target the sheet does not host becomes the tab it would have opened as.
+            const promoted = promoteSheetToTab(next, normalizedWorkspaceKey);
+            set(promoted?.patch ?? withoutSheet(next, normalizedWorkspaceKey));
+            return promoted?.tabId ?? null;
+          }
           const result = replaceTabTargetInLayout({
             layout: getWorkspaceLayout(get().layoutByWorkspace, normalizedWorkspaceKey),
             tabId: normalizedTabId,
@@ -1054,6 +1378,19 @@ export function createWorkspaceLayoutStore(
           const normalizedTabId = trimNonEmpty(tabId);
           if (!normalizedWorkspaceKey || !normalizedTabId) return;
           set((state) => {
+            const sheet = state.sheetByWorkspace[normalizedWorkspaceKey];
+            if (sheet?.tab.tabId === normalizedTabId) {
+              const { state: _previous, ...tab } = sheet.tab;
+              return {
+                sheetByWorkspace: {
+                  ...state.sheetByWorkspace,
+                  [normalizedWorkspaceKey]: {
+                    ...sheet,
+                    tab: tabState === undefined ? tab : { ...tab, state: tabState },
+                  },
+                },
+              };
+            }
             const layout = setTabStateInLayout({
               layout: getWorkspaceLayout(state.layoutByWorkspace, normalizedWorkspaceKey),
               tabId: normalizedTabId,
@@ -1148,15 +1485,22 @@ export function createWorkspaceLayoutStore(
                 );
               }
             }
+            const sheetAgentGone = sheetAgentNoLongerFits(
+              normalizedWorkspaceKey,
+              state.sheetByWorkspace[normalizedWorkspaceKey],
+              snapshot,
+            );
             if (
               state.layoutByWorkspace[normalizedWorkspaceKey] &&
               nextLayout === rawLayout &&
-              pinnedAgentIdsByWorkspace === state.pinnedAgentIdsByWorkspace
+              pinnedAgentIdsByWorkspace === state.pinnedAgentIdsByWorkspace &&
+              !sheetAgentGone
             ) {
               return state;
             }
 
             return {
+              ...(sheetAgentGone ? withoutSheet(state, normalizedWorkspaceKey) : {}),
               pinnedAgentIdsByWorkspace,
               layoutByWorkspace: {
                 ...state.layoutByWorkspace,
@@ -1661,7 +2005,8 @@ export function createWorkspaceLayoutStore(
               normalizedWorkspaceKey in state.focusRestorationByWorkspace ||
               normalizedWorkspaceKey in state.explorerSidebarPaneIdByWorkspace ||
               normalizedWorkspaceKey in state.sidePaneIdByWorkspace ||
-              normalizedWorkspaceKey in state.pullRequestTabAutoOpenedByWorkspace;
+              normalizedWorkspaceKey in state.pullRequestTabAutoOpenedByWorkspace ||
+              normalizedWorkspaceKey in state.sheetByWorkspace;
             if (!hasAny) {
               return state;
             }
@@ -1690,6 +2035,7 @@ export function createWorkspaceLayoutStore(
             const { [normalizedWorkspaceKey]: _sidePane, ...sidePaneIdByWorkspace } =
               state.sidePaneIdByWorkspace;
             return {
+              ...withoutSheet(state, normalizedWorkspaceKey),
               pullRequestTabAutoOpenedByWorkspace,
               layoutByWorkspace,
               splitSizesByWorkspace,
@@ -1795,10 +2141,10 @@ export function createWorkspaceLayoutStore(
 export const useWorkspaceLayoutStore = createWorkspaceLayoutStore();
 
 /**
- * The agent tabs that exist right now, across every workspace of this host, independently
+ * The agent tabs that exist right now, including sheet tabs, across every workspace of this host,
- * of which workspace views are mounted. Timeline sync uses this to release a chat when its
+ * independently of which workspace views are mounted. Timeline sync uses this to release a chat
- * tab closes. It is not a subscription source: this layout is restored from disk at launch,
+ * when its tab closes. It is not a subscription source: this layout is restored from disk at
- * so subscribing to it would resume every agent the user has ever opened.
+ * launch, so subscribing to it would resume every agent the user has ever opened.
  */
 export function observeOpenWorkspaceAgentIds(
   serverId: string,
@@ -1806,11 +2152,19 @@ export function observeOpenWorkspaceAgentIds(
   store = useWorkspaceLayoutStore,
 ): () => void {
   let previous: string[] | undefined;
-  const publish = (state: Pick<WorkspaceLayoutStore, "layoutByWorkspace">) => {
+  const publish = (state: Pick<WorkspaceLayoutStore, "layoutByWorkspace" | "sheetByWorkspace">) => {
     const ids = new Set<string>();
-    for (const [workspaceKey, layout] of Object.entries(state.layoutByWorkspace)) {
+    const tabsByWorkspace = [
+      ...Object.entries(state.layoutByWorkspace).map(
+        ([workspaceKey, layout]) => [workspaceKey, collectAllTabs(layout.root)] as const,
+      ),
+      ...Object.entries(state.sheetByWorkspace).map(
+        ([workspaceKey, sheet]) => [workspaceKey, [sheet.tab]] as const,
+      ),
+    ];
+    for (const [workspaceKey, tabs] of tabsByWorkspace) {
       if (!workspaceKey.startsWith(`${serverId}:`)) continue;
-      for (const tab of collectAllTabs(layout.root)) {
+      for (const tab of tabs) {
         if (tab.target.kind === "agent") ids.add(tab.target.agentId);
       }
     }
@@ -1821,7 +2175,12 @@ export function observeOpenWorkspaceAgentIds(
     listener(next);
   };
   const unsubscribe = store.subscribe((state, before) => {
-    if (state.layoutByWorkspace !== before.layoutByWorkspace) publish(state);
+    if (
+      state.layoutByWorkspace !== before.layoutByWorkspace ||
+      state.sheetByWorkspace !== before.sheetByWorkspace
+    ) {
+      publish(state);
+    }
   });
   publish(store.getState());
   return unsubscribe;

packages/app/src/workspace-tabs/compact-sheet.ts

@@ -0,0 +1,38 @@
+import { getWorkspacePanelAttributes } from "@/panels/panel-instance-attributes";
+import { panelCompactSheetPolicy, type CompactSheetPolicy } from "@/panels/panel-manifest";
+import { useSessionStore, type Agent } from "@/stores/session-store";
+import { isWorkspaceRootAgent } from "@/subagents/policies";
+import type { WorkspaceTabTarget } from "@/workspace-tabs/model";
+
+function workspaceAgentLookup(workspaceKey: string): (agentId: string) => Agent | null {
+  const sessions = useSessionStore.getState().sessions;
+  const serverId = Object.keys(sessions)
+    .filter((candidate) => workspaceKey.startsWith(`${candidate}:`))
+    .sort((left, right) => right.length - left.length)[0];
+  const session = serverId ? sessions[serverId] : undefined;
+  return (agentId) => session?.agents.get(agentId) ?? session?.agentDetails.get(agentId) ?? null;
+}
+
+/**
+ * The single target-aware boundary deciding whether a compact reveal lands in the workspace
+ * sheet. An agent the client has not loaded yet counts as a root agent and stays a tab.
+ */
+export function resolveCompactSheetPolicy(
+  workspaceKey: string,
+  target: WorkspaceTabTarget,
+): CompactSheetPolicy | null {
+  return panelCompactSheetPolicy(target, {
+    isWorkspaceRootAgent: (agentId) => {
+      const getAgent = workspaceAgentLookup(workspaceKey);
+      const agent = getAgent(agentId);
+      if (!agent) return true;
+      const parent = agent.parentAgentId ? (getAgent(agent.parentAgentId) ?? undefined) : undefined;
+      return isWorkspaceRootAgent(agent, parent);
+    },
+  });
+}
+
+/** Whether the sheet's tab holds unsaved work, which must never be discarded with the sheet. */
+export function isSheetTabModified(workspaceKey: string, tabId: string): boolean {
+  return getWorkspacePanelAttributes(workspaceKey, tabId).modified;
+}

packages/app/src/workspace-tabs/layout-mode-publisher.tsx

@@ -0,0 +1,21 @@
+import { useLayoutEffect } from "react";
+import { supportsDesktopPaneSplits, useIsCompactFormFactor } from "@/constants/layout";
+import { useWorkspaceLayoutStore, type WorkspaceLayoutMode } from "@/stores/workspace-layout-store";
+
+function resolveWorkspaceLayoutMode(isCompact: boolean): WorkspaceLayoutMode {
+  if (isCompact) return "compact";
+  return supportsDesktopPaneSplits() ? "split-panes" : "single-pane";
+}
+
+/**
+ * Publishes the shell's workspace layout to the layout store, which decides where opened targets
+ * land. Render it before the routes: React runs an earlier sibling's layout effects before a later
+ * sibling's subtree, so a route sees the mode even from its first layout effect.
+ */
+export function WorkspaceLayoutModePublisher(): null {
+  const isCompact = useIsCompactFormFactor();
+  useLayoutEffect(() => {
+    useWorkspaceLayoutStore.getState().setLayoutMode(resolveWorkspaceLayoutMode(isCompact));
+  }, [isCompact]);
+  return null;
+}

packages/app/src/workspace-tabs/open-beside.test.ts

@@ -0,0 +1,177 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+vi.mock("@react-native-async-storage/async-storage", () => ({
+  default: {
+    getItem: vi.fn(async () => null),
+    setItem: vi.fn(async () => undefined),
+    removeItem: vi.fn(async () => undefined),
+  },
+}));
+
+import { DEFAULT_APP_SETTINGS } from "@/hooks/use-settings";
+import { setPanelInstanceAttributes } from "@/panels/panel-instance-attributes";
+import {
+  collectAllPanes,
+  collectAllTabs,
+  findPaneById,
+  useWorkspaceLayoutStore,
+} from "@/stores/workspace-layout-store";
+import {
+  openPreferredWorkspacePreview,
+  openPreferredWorkspaceTarget,
+  openWorkspaceTargetBeside,
+} from "@/workspace-tabs/open-beside";
+
+const WORKSPACE_KEY = "server-1:workspace-1";
+
+beforeEach(() => {
+  useWorkspaceLayoutStore.setState({
+    layoutMode: "split-panes",
+    sheetByWorkspace: {},
+    layoutByWorkspace: {},
+    explorerSidebarPaneIdByWorkspace: {},
+    sidePaneIdByWorkspace: {},
+  });
+});
+
+describe("openPreferredWorkspacePreview", () => {
+  it("opens a compact file preview in the sheet instead of replacing the active file tab", () => {
+    const store = useWorkspaceLayoutStore.getState();
+    const fileTabId = store.openTab({
+      workspaceKey: WORKSPACE_KEY,
+      target: { kind: "file", path: "src/open.ts" },
+      intent: "reveal",
+    });
+    store.setLayoutMode("compact");
+
+    const tabId = openPreferredWorkspacePreview({
+      workspaceKey: WORKSPACE_KEY,
+      serverId: "server-1",
+      workspaceId: "workspace-1",
+      explorerSidebarPaneId: "explorer",
+      lastMainPaneId: "main",
+      target: { kind: "file", path: "src/preview.ts" },
+      source: "explorerFiles",
+      preferences: DEFAULT_APP_SETTINGS.openInSidePane,
+    });
+
+    const state = useWorkspaceLayoutStore.getState();
+    expect(state.sheetByWorkspace[WORKSPACE_KEY]?.tab).toMatchObject({
+      tabId,
+      target: { kind: "file", path: "src/preview.ts" },
+    });
+    const root = state.layoutByWorkspace[WORKSPACE_KEY].root;
+    expect(findPaneById(root, "main")?.tabIds).toEqual([fileTabId]);
+    expect(collectAllTabs(root).find((tab) => tab.tabId === fileTabId)?.target).toEqual({
+      kind: "file",
+      path: "src/open.ts",
+    });
+  });
+});
+
+describe("previews under a sheet with unsaved work", () => {
+  const sheetTarget = { kind: "file" as const, path: "src/draft.ts" };
+  let sheetTabId: string | null = null;
+
+  afterEach(() => {
+    if (sheetTabId) {
+      setPanelInstanceAttributes(
+        { serverId: "server-1", workspaceId: "workspace-1", tabId: sheetTabId },
+        { modified: false },
+      );
+    }
+    sheetTabId = null;
+  });
+
+  function previewFile(path: string): string | null {
+    return openPreferredWorkspacePreview({
+      workspaceKey: WORKSPACE_KEY,
+      serverId: "server-1",
+      workspaceId: "workspace-1",
+      explorerSidebarPaneId: "explorer",
+      lastMainPaneId: "main",
+      target: { kind: "file", path },
+      source: "explorerFiles",
+      preferences: DEFAULT_APP_SETTINGS.openInSidePane,
+    });
+  }
+
+  it("reuses one preview tab instead of adding a tab per link", () => {
+    const store = useWorkspaceLayoutStore.getState();
+    store.openTab({
+      workspaceKey: WORKSPACE_KEY,
+      target: { kind: "terminal", terminalId: "term-1" },
+      intent: "reveal",
+    });
+    store.setLayoutMode("compact");
+    sheetTabId = store.openTab({
+      workspaceKey: WORKSPACE_KEY,
+      target: sheetTarget,
+      intent: "reveal",
+    });
+    setPanelInstanceAttributes(
+      { serverId: "server-1", workspaceId: "workspace-1", tabId: sheetTabId ?? "" },
+      { modified: true },
+    );
+
+    const firstPreviewTabId = previewFile("src/first.ts");
+    const secondPreviewTabId = previewFile("src/second.ts");
+
+    const state = useWorkspaceLayoutStore.getState();
+    expect(state.sheetByWorkspace[WORKSPACE_KEY]?.tab.target).toEqual(sheetTarget);
+    expect(secondPreviewTabId).toBe(firstPreviewTabId);
+    const root = state.layoutByWorkspace[WORKSPACE_KEY].root;
+    const tabsById = new Map(collectAllTabs(root).map((tab) => [tab.tabId, tab.target]));
+    expect(findPaneById(root, "main")?.tabIds.map((tabId) => tabsById.get(tabId))).toEqual([
+      { kind: "terminal", terminalId: "term-1" },
+      { kind: "file", path: "src/second.ts" },
+    ]);
+  });
+});
+
+describe("opening beside", () => {
+  const sidePreference = { ...DEFAULT_APP_SETTINGS.openInSidePane, subagents: true };
+
+  function openSubagentWithSidePreference(): string | null {
+    useWorkspaceLayoutStore.getState().openTab({
+      workspaceKey: WORKSPACE_KEY,
+      target: { kind: "file", path: "src/open.ts" },
+      intent: "reveal",
+    });
+    return openPreferredWorkspaceTarget({
+      workspaceKey: WORKSPACE_KEY,
+      target: { kind: "file", path: "src/subagent.ts" },
+      source: "subagents",
+      preferences: sidePreference,
+    });
+  }
+
+  function paneIdHolding(tabId: string | null): string | undefined {
+    const root = useWorkspaceLayoutStore.getState().layoutByWorkspace[WORKSPACE_KEY].root;
+    return collectAllPanes(root).find((pane) => pane.tabIds.includes(tabId ?? ""))?.id;
+  }
+
+  it("follows the side preference where the workspace renders split panes", () => {
+    useWorkspaceLayoutStore.getState().setLayoutMode("split-panes");
+
+    const tabId = openSubagentWithSidePreference();
+
+    const sidePaneId = useWorkspaceLayoutStore.getState().sidePaneIdByWorkspace[WORKSPACE_KEY];
+    expect(sidePaneId).toEqual(expect.any(String));
+    expect(paneIdHolding(tabId)).toBe(sidePaneId);
+  });
+
+  it("keeps a single-pane workspace in one pane despite the side preference", () => {
+    useWorkspaceLayoutStore.getState().setLayoutMode("single-pane");
+
+    const preferredTabId = openSubagentWithSidePreference();
+    const besideTabId = openWorkspaceTargetBeside({
+      workspaceKey: WORKSPACE_KEY,
+      target: { kind: "file", path: "src/beside.ts" },
+    });
+
+    expect(useWorkspaceLayoutStore.getState().sidePaneIdByWorkspace[WORKSPACE_KEY]).toBeFalsy();
+    expect(paneIdHolding(preferredTabId)).toBe("main");
+    expect(paneIdHolding(besideTabId)).toBe("main");
+  });
+});

packages/app/src/workspace-tabs/open-beside.ts

@@ -22,13 +22,11 @@ interface OpenWorkspaceTargetInput {
 }
 
 export interface OpenPreferredWorkspaceTargetInput extends OpenWorkspaceTargetInput {
-  isCompact: boolean;
   source: OpenInSidePaneSource;
   preferences: OpenInSidePanePreferences;
 }
 
 interface OpenWorkspaceTargetAtLocationInput extends OpenWorkspaceTargetInput {
-  isCompact: boolean;
   location: WorkspaceTargetOpenLocation;
 }
 
@@ -39,6 +37,11 @@ interface OpenPreferredWorkspacePreviewInput extends OpenPreferredWorkspaceTarge
   lastMainPaneId: string | null;
 }
 
+/** A side pane exists only where the workspace renders split panes. */
+function canOpenBeside(): boolean {
+  return useWorkspaceLayoutStore.getState().layoutMode === "split-panes";
+}
+
 function resolveMainPane(input: {
   workspaceKey: string;
   explorerSidebarPaneId: string | null;
@@ -86,7 +89,6 @@ export function openPreferredWorkspaceTarget(
   input: OpenPreferredWorkspaceTargetInput,
 ): string | null {
   return openWorkspaceTargetAtLocation({
-    isCompact: input.isCompact,
     workspaceKey: input.workspaceKey,
     target: input.target,
     location: input.preferences[input.source] ? "side" : "main",
@@ -105,7 +107,7 @@ export function openWorkspaceTargetAtLocation(
     layout &&
     collectAllTabs(layout.root).some((tab) => workspaceTabTargetsEqual(tab.target, input.target)),
   );
-  const shouldOpenBeside = !input.isCompact && input.location === "side";
+  const shouldOpenBeside = canOpenBeside() && input.location === "side";
   let placement: WorkspaceTabPlacement | undefined;
   if (shouldOpenBeside && !targetAlreadyExists) {
     const paneId = store.ensureSidePane(input.workspaceKey);
@@ -135,7 +137,13 @@ export function openPreferredWorkspacePreview(
   const existing = collectAllTabs(layout.root).some((tab) =>
     workspaceTabTargetsEqual(tab.target, input.target),
   );
-  if (existing) return openPreferredWorkspaceTarget(input);
+  // The sheet is already a single replaceable slot, so it needs no preview tab.
+  if (
+    existing ||
+    store.opensInSheet({ workspaceKey: input.workspaceKey, target: input.target, intent: "reveal" })
+  ) {
+    return openPreferredWorkspaceTarget(input);
+  }
 
   const mainPane = resolveMainPane({
     workspaceKey: input.workspaceKey,
@@ -143,7 +151,7 @@ export function openPreferredWorkspacePreview(
     lastMainPaneId: input.lastMainPaneId,
   });
   const destinationPaneId =
-    !input.isCompact && input.preferences[input.source]
+    canOpenBeside() && input.preferences[input.source]
       ? store.ensureSidePane(input.workspaceKey)
       : mainPane?.id;
   if (!destinationPaneId) return null;
@@ -173,9 +181,15 @@ export function openPreferredWorkspacePreview(
   });
 }
 
-/** Explicit Open to Side: creates the side pane and places the target there. */
+/**
+ * Explicit Open to Side: creates the side pane and places the target there. Without split panes
+ * the target opens like any other.
+ */
 export function openWorkspaceTargetBeside(input: OpenWorkspaceTargetInput): string | null {
   if (!input.workspaceKey) return null;
+  if (!canOpenBeside()) {
+    return openWorkspaceTargetAtLocation({ ...input, location: "main" });
+  }
   const store = useWorkspaceLayoutStore.getState();
   const paneId = store.ensureSidePane(input.workspaceKey);
   return store.openTab({

packages/app/src/workspace-tabs/open-supporting-view.ts

@@ -46,7 +46,6 @@ export function openWorkspaceChanges(input: OpenWorkspaceChangesInput): string |
     return null;
   }
   return openPreferredWorkspaceTarget({
-    isCompact: input.isCompact,
     workspaceKey: input.workspaceKey,
     target: { kind: "working_diff" },
     source: "diffs",
@@ -70,7 +69,6 @@ export function openWorkspacePullRequest(input: OpenWorkspacePullRequestInput):
     return null;
   }
   return openWorkspaceTargetAtLocation({
-    isCompact: input.isCompact,
     workspaceKey: input.workspaceKey,
     target: { kind: "pull_request" },
     location: input.destination,