diff --git a/docs/coding-standards.md b/docs/coding-standards.md
index ec77d103b8..9d932b32dc 100644
--- a/docs/coding-standards.md
+++ b/docs/coding-standards.md
@@ -98,6 +98,14 @@ For testing rules, see [testing.md](testing.md).
 - Use stable ids for `key`, never array index for reorderable/filterable lists.
 - Context for stable values (theme, auth). Store with selectors for state that changes.
 
+### Retained panel measurements
+
+Inactive retained panels use `display: none`, so DOM and layout APIs report zero width and height. A non-positive measurement means the surface is not rendered; it is never a valid compact layout.
+
+- Suspend geometry-derived updates while either measured dimension is non-positive. Preserve the last valid state until the surface reports positive geometry again.
+- Enforce the rule at the measurement boundary shared by every trigger. Guarding only an `onLayout` or `ResizeObserver` callback still lets effects, content changes, and imperative remeasurement publish hidden geometry.
+- Test the hide-and-reveal path through a real retained surface. Settled assertions do not catch stale geometry painted during the first returned frames.
+
 ## Naming
 
 - Names describe meaning, not mechanics. `submitForm` over `handleOnClickButtonSubmit`. `running` over `filteredArrayOfRunningAgents`.
diff --git a/docs/expo-router.md b/docs/expo-router.md
index 2f586fd801..3287d6a7d4 100644
--- a/docs/expo-router.md
+++ b/docs/expo-router.md
@@ -67,7 +67,9 @@ that decision; if no host route is mounted yet, it falls back to ordinary route
 navigation.
 
 Those hidden entries are not harmless: composer floating panels can measure
-against the wrong deck and disappear offscreen.
+against the wrong deck and disappear offscreen. Follow the
+[retained panel measurement rules](coding-standards.md#retained-panel-measurements)
+for components that intentionally remain mounted while hidden.
 
 Hidden host routes may keep their local params while an app-wide route is
 foregrounded. Active-workspace observers must prefer the current pathname and
diff --git a/packages/app/e2e/browser/settings-toggle-tab-regression.spec.ts b/packages/app/e2e/browser/settings-toggle-tab-regression.spec.ts
index f1abdd0906..5b52c7025d 100644
--- a/packages/app/e2e/browser/settings-toggle-tab-regression.spec.ts
+++ b/packages/app/e2e/browser/settings-toggle-tab-regression.spec.ts
@@ -10,6 +10,58 @@ async function pressSettingsToggleShortcut(page: import("@playwright/test").Page
   await page.keyboard.press(`${modifier}+Comma`);
 }
 
+interface ComposerFrame {
+  at: number;
+  visible: boolean;
+  rootHeight: number;
+  inputHeight: number;
+  inputWidth: number;
+  inputStyleHeight: string;
+  inputStyleMaxHeight: string;
+}
+
+async function startComposerFrameCapture(page: import("@playwright/test").Page) {
+  await page.evaluate(() => {
+    const capture = { frames: [] as ComposerFrame[], stopped: false };
+    Object.assign(window, { __composerFrameCapture: capture });
+
+    const sample = () => {
+      if (capture.stopped) return;
+      const root = document.querySelector<HTMLElement>('[data-testid="message-input-root"]');
+      const input = root?.querySelector<HTMLTextAreaElement>("textarea");
+      if (root && input) {
+        const style = getComputedStyle(input);
+        capture.frames.push({
+          at: performance.now(),
+          visible: root.offsetParent !== null,
+          rootHeight: root.getBoundingClientRect().height,
+          inputHeight: input.getBoundingClientRect().height,
+          inputWidth: input.clientWidth,
+          inputStyleHeight: style.height,
+          inputStyleMaxHeight: style.maxHeight,
+        });
+      }
+      requestAnimationFrame(sample);
+    };
+    requestAnimationFrame(sample);
+  });
+}
+
+async function stopComposerFrameCapture(
+  page: import("@playwright/test").Page,
+): Promise<ComposerFrame[]> {
+  return page.evaluate(() => {
+    const capture = (
+      window as typeof window & {
+        __composerFrameCapture?: { frames: ComposerFrame[]; stopped: boolean };
+      }
+    ).__composerFrameCapture;
+    if (!capture) throw new Error("Composer frame capture was not installed");
+    capture.stopped = true;
+    return capture.frames;
+  });
+}
+
 async function expectSendBehavior(
   page: import("@playwright/test").Page,
   expected: "interrupt" | "queue" | "steer",
@@ -107,6 +159,65 @@ test.describe("Settings toggle tab regression", () => {
     }
   });
 
+  test("returning from settings does not flash the composer at maximum height", async ({
+    page,
+  }) => {
+    const workspace = await seedWorkspace({ repoPrefix: "settings-composer-height-" });
+
+    try {
+      const firstAgent = await createIdleAgent(workspace.client, {
+        cwd: workspace.repoPath,
+        workspaceId: workspace.workspaceId,
+        title: `settings-composer-height-a-${Date.now()}`,
+      });
+      const secondAgent = await createIdleAgent(workspace.client, {
+        cwd: workspace.repoPath,
+        workspaceId: workspace.workspaceId,
+        title: `settings-composer-height-b-${Date.now()}`,
+      });
+      await openWorkspaceWithAgents(page, [firstAgent, secondAgent]);
+      await waitForTabBar(page);
+
+      const input = page.getByRole("textbox", { name: "Message agent..." });
+      await input.fill("Keep this short draft in the composer");
+      const root = page.getByTestId("message-input-root").filter({ visible: true }).first();
+      const baselineHeight = (await root.boundingBox())?.height;
+      expect(baselineHeight).toBeDefined();
+
+      await startComposerFrameCapture(page);
+      await pressSettingsToggleShortcut(page);
+      await expect(page).toHaveURL(/\/settings\/general$/);
+      await pressSettingsToggleShortcut(page);
+      await expect(page).not.toHaveURL(/\/settings(\/|$)/);
+      await expect(input).toHaveValue("Keep this short draft in the composer");
+      await page.waitForTimeout(250);
+
+      const frames = await stopComposerFrameCapture(page);
+      const visibleFrames = frames.filter((frame) => frame.visible);
+      const tallestIndex = visibleFrames.reduce(
+        (index, frame, candidateIndex) =>
+          frame.rootHeight > visibleFrames[index].rootHeight ? candidateIndex : index,
+        0,
+      );
+      const tallestFrame = visibleFrames[tallestIndex];
+      const tallestFrameIndex = frames.findIndex((frame) => frame.at === tallestFrame.at);
+      expect(
+        tallestFrame.rootHeight,
+        JSON.stringify(
+          {
+            baselineHeight,
+            tallestFrame,
+            nearbyFrames: frames.slice(Math.max(0, tallestFrameIndex - 3), tallestFrameIndex + 4),
+          },
+          null,
+          2,
+        ),
+      ).toBeLessThanOrEqual((baselineHeight ?? 0) + 5);
+    } finally {
+      await workspace.cleanup();
+    }
+  });
+
   test("refresh after navigating between agent routes keeps the latest agent focused", async ({
     page,
   }) => {
diff --git a/packages/app/src/composer/input/height.web.ts b/packages/app/src/composer/input/height.web.ts
index 2dd5982d4b..4e3bef363d 100644
--- a/packages/app/src/composer/input/height.web.ts
+++ b/packages/app/src/composer/input/height.web.ts
@@ -56,12 +56,14 @@ export function useComposerHeight({
       const mirror = mirrorRef.current;
       const source = textareaRef.current;
       if (!mirror || !source || typeof window === "undefined") return;
+      const sourceWidth = source.clientWidth;
+      if (sourceWidth <= 0) return;
 
       const computedStyle = window.getComputedStyle(source);
       for (const property of COPIED_STYLES) {
         mirror.style[property] = computedStyle[property];
       }
-      mirror.style.width = `${source.clientWidth}px`;
+      mirror.style.width = `${sourceWidth}px`;
       mirror.value = text.endsWith("\n") ? `${text} ` : text;
       setBoundedHeight(mirror.scrollHeight);
     },