fix(app): avoid pane rerenders while resizing Explorer (#7284)

Keep drag widths out of the shared stylesheet and skip unchanged panes during width previews. Preserve the existing resize state and persistence behavior.

Co-authored-by: paseo-bot[bot] <266920839+paseo-bot[bot]@users.noreply.github.com>
Co-authored-by: Ivan Malison <1246619+colonelpanic8@users.noreply.github.com>

packages/app/e2e/browser/explorer-resize-performance.spec.ts

@@ -0,0 +1,118 @@
+import type { Page } from "@playwright/test";
+import { test, expect } from "../support/fixtures";
+import { runWorkspaceActionFromCommandCenter } from "../support/helpers/command-center-workspace-actions";
+
+interface DragMeasurement {
+  observer: MutationObserver;
+  stylesheetMutations: number;
+}
+
+declare global {
+  interface Window {
+    __PASEO_EXPLORER_DRAG_MEASUREMENT__?: DragMeasurement;
+  }
+}
+
+async function waitForResizePaint(page: Page): Promise<void> {
+  await page.evaluate(
+    () =>
+      new Promise<void>((resolve) => {
+        requestAnimationFrame(() => requestAnimationFrame(() => resolve()));
+      }),
+  );
+}
+
+async function beginResizeMeasurement(page: Page): Promise<void> {
+  await page.evaluate(() => {
+    const stylesheet = document.getElementById("unistyles-web");
+    if (!stylesheet) throw new Error("Shared stylesheet is missing");
+    globalThis.__PASEO_RESET_RENDER_PROFILE__!();
+    const measurement: DragMeasurement = {
+      stylesheetMutations: 0,
+      observer: new MutationObserver((records) => {
+        measurement.stylesheetMutations += records.length;
+      }),
+    };
+    measurement.observer.observe(stylesheet, {
+      childList: true,
+      subtree: true,
+      characterData: true,
+    });
+    window.__PASEO_EXPLORER_DRAG_MEASUREMENT__ = measurement;
+  });
+}
+
+async function previewExplorerResize(page: Page, distance: number) {
+  const handle = page
+    .getByTestId("workspace-explorer-sidebar-resize-handle")
+    .getByRole("separator");
+  const explorer = page.getByTestId("workspace-explorer-sidebar");
+  const bounds = await handle.boundingBox();
+  if (!bounds) throw new Error("Explorer resize handle is missing");
+  const before = (await explorer.boundingBox())!.width;
+  const x = bounds.x + bounds.width / 2;
+  const y = bounds.y + 100;
+  await page.mouse.move(x, y);
+  await page.mouse.down();
+  await waitForResizePaint(page);
+  await beginResizeMeasurement(page);
+  for (let move = 1; move <= 20; move++) {
+    await page.mouse.move(x + (distance * move) / 20, y);
+    await waitForResizePaint(page);
+  }
+  const during = (await explorer.boundingBox())!.width;
+  const measurement = await page.evaluate(() => {
+    const state = window.__PASEO_EXPLORER_DRAG_MEASUREMENT__!;
+    state.observer.disconnect();
+    return {
+      stylesheetMutations: state.stylesheetMutations,
+      paneRenders: Object.entries(globalThis.__PASEO_RENDER_PROFILE_REASONS__ ?? {})
+        .filter(([id]) => id.startsWith("SplitPaneView:"))
+        .reduce((sum, [, reasons]) => sum + (reasons.render ?? 0), 0),
+    };
+  });
+  await page.mouse.up();
+  await waitForResizePaint(page);
+  const after = (await explorer.boundingBox())!.width;
+  expect(during).toBeCloseTo(before - distance, 0);
+  expect(after).toBeCloseTo(during, 0);
+  return { ...measurement, width: after };
+}
+
+async function expectExplorerWidthSurvivesReload(page: Page, width: number): Promise<void> {
+  await page.reload();
+  const explorer = page.getByTestId("workspace-explorer-sidebar");
+  await expect(explorer).toBeVisible();
+  await expect.poll(async () => (await explorer.boundingBox())?.width).toBeCloseTo(width, 0);
+}
+
+test("Explorer drag previews without rerendering panes or rewriting the shared stylesheet", async ({
+  page,
+  withWorkspace,
+}) => {
+  await page.setViewportSize({ width: 1600, height: 1000 });
+  await page.addInitScript(() => {
+    globalThis.__PASEO_RENDER_PROFILE_ENABLED__ = true;
+  });
+  const workspace = await withWorkspace();
+  await workspace.navigateTo();
+  await runWorkspaceActionFromCommandCenter(page, "Split pane right");
+  await runWorkspaceActionFromCommandCenter(page, "Split pane right");
+  await page.getByTestId("workspace-explorer-toggle").first().click();
+  await expect(page.getByTestId("workspace-explorer-sidebar")).toBeVisible();
+  // Verify that direct pane renders are being counted before resetting them.
+  expect(
+    await page.evaluate(() =>
+      Object.keys(globalThis.__PASEO_RENDER_PROFILE_REASONS__ ?? {}).some((id) =>
+        id.startsWith("SplitPaneView:"),
+      ),
+    ),
+  ).toBe(true);
+  const grown = await previewExplorerResize(page, -100);
+  const shrunk = await previewExplorerResize(page, 50);
+  expect(grown.stylesheetMutations).toBe(0);
+  expect(grown.paneRenders).toBe(0);
+  expect(shrunk.stylesheetMutations).toBe(0);
+  expect(shrunk.paneRenders).toBe(0);
+  await expectExplorerWidthSurvivesReload(page, shrunk.width);
+});

packages/app/src/components/split-container.tsx

@@ -1,5 +1,6 @@
 import {
   Fragment,
+  memo,
   useCallback,
   useEffect,
   useMemo,
@@ -88,7 +89,8 @@ import {
   type WorkspaceLayout,
 } from "@/stores/workspace-layout-store";
 import type { WorkspaceTab } from "@/workspace-tabs/model";
-import { RenderProfile } from "@/utils/render-profiler";
+import { recordRenderProfileReasons, RenderProfile } from "@/utils/render-profiler";
+import { inlineUnistylesStyle } from "@/styles/unistyles-inline-style";
 import { isNative } from "@/constants/platform";
 import { panelTargetSupportsHost } from "@/plugins/workspace-panels/locations";
 
@@ -435,7 +437,7 @@ export function SplitContainer({
     : inheritedWindowChromeCorners;
   const mainColumnStyle = styles.mainColumn;
   const explorerSidebarDockStyle = useMemo(
-    () => [styles.explorerSidebarDock, { width: explorerSidebarWidth }],
+    () => [styles.explorerSidebarDock, inlineUnistylesStyle({ width: explorerSidebarWidth })],
     [explorerSidebarWidth],
   );
   const handleWorkspaceShellLayout = useCallback((event: LayoutChangeEvent) => {
@@ -1121,7 +1123,7 @@ function SplitNodeView({
   );
 }
 
-function SplitPaneView({
+const SplitPaneView = memo(function SplitPaneView({
   pane,
   uiTabs,
   isFocused,
@@ -1158,6 +1160,7 @@ function SplitPaneView({
   focusModeEnabled,
   onExitFocusMode,
 }: SplitPaneViewProps) {
+  recordRenderProfileReasons(`SplitPaneView:${pane.id}`, ["render"]);
   const paneRef = useRef<View | null>(null);
   const stableOnFocusPane = useStableEvent(onFocusPane);
   const paneState = useMemo(
@@ -1312,7 +1315,7 @@ function SplitPaneView({
       </View>
     </RenderProfile>
   );
-}
+});
 
 function collectPanesById(node: SplitNode): Map<string, SplitPane> {
   const next = new Map<string, SplitPane>();