perf(web): thread switches skip re-parsing, sidebar scrolling, and composer layout reads

apps/web/src/components/ChatMarkdown.browser.test.tsx

@@ -16,6 +16,18 @@ const { workspaceSearch, filesystemMetadata } = vi.hoisted(() => ({
   workspaceSearch: vi.fn(),
   filesystemMetadata: vi.fn(),
 }));
+const parsedSources = vi.hoisted(() => [] as string[]);
+vi.mock(import("mdast-util-from-markdown"), async (original) => {
+  const actual = await original();
+  return {
+    ...actual,
+    fromMarkdown: ((source: string, ...rest: never[]) => {
+      parsedSources.push(String(source));
+      return (actual.fromMarkdown as (...args: unknown[]) => unknown)(source, ...rest);
+    }) as typeof actual.fromMarkdown,
+  };
+});
+
 vi.mock(import("../state/use-atom-query-runner"), async (original) => ({
   ...(await original()),
   useAtomQueryRunner: ((family: unknown) =>
@@ -139,6 +151,20 @@ it("loads a host image through the page's own route when there is no environment
   );
 });
 
+it("reuses the parse of a streaming reply's finished blocks when it remounts", async () => {
+  setup();
+  parsedSources.length = 0;
+  const message = (
+    <ChatMarkdown cwd="/tmp/project" text={"Finished block\n\nStill growing"} isStreaming />
+  );
+  {
+    await using _first = await render(message);
+  }
+  await using screen = await render(message);
+  await expect.element(screen.getByText("Still growing")).toBeVisible();
+  expect(parsedSources.filter((source) => source.includes("Finished block"))).toHaveLength(1);
+});
+
 describe("ChatMarkdown favicon privacy", () => {
   it("suppresses private link images while preserving public links across updates", async () => {
     setup();

apps/web/src/components/ChatMarkdown.tsx

@@ -1767,7 +1767,8 @@ function ChatMarkdown({
                 : CHAT_MARKDOWN_LITERAL_HTML_REHYPE_PLUGINS
             }
             shared={!uncachedPlugins}
-            cache={!props.isStreaming}
+            // Every block before the last is finished, so its parse can be reused.
+            cache={!props.isStreaming || index < blocks.length - 1}
           />
         ))}
       </ChatMarkdownRendererContext>

apps/web/src/components/chat/useComposerMultilinePrompt.browser.test.tsx

@@ -0,0 +1,23 @@
+import { expect, it } from "vite-plus/test";
+
+import { render } from "../../../test/render";
+import { useComposerMultilinePrompt } from "./useComposerMultilinePrompt";
+
+function Composer({ text }: { readonly text: string }) {
+  const [multiline, observeBody] = useComposerMultilinePrompt();
+  return (
+    <div ref={observeBody} style={{ width: 200, lineHeight: "20px" }}>
+      <div data-testid="composer-editor" contentEditable suppressContentEditableWarning>
+        {text}
+      </div>
+      <output>{multiline ? "multiline" : "single line"}</output>
+    </div>
+  );
+}
+
+it("notices when typing makes the prompt wrap", async () => {
+  await using screen = await render(<Composer text="short" />, { router: false });
+  await expect.element(screen.getByText("single line")).toBeVisible();
+  await screen.rerender(<Composer text={"a long prompt that wraps ".repeat(6)} />);
+  await expect.element(screen.getByText("multiline")).toBeVisible();
+});

apps/web/src/components/chat/useComposerMultilinePrompt.ts

@@ -27,16 +27,24 @@ export function useComposerMultilinePrompt(): readonly [
   const [isMultiline, setIsMultiline] = useState(false);
   const observeBody = (body: HTMLElement | null) => {
     if (!body) return;
+    // Measuring reads layout, so one measurement per frame runs after all of the frame's DOM
+    // writes instead of forcing a layout after each mutation.
+    let frame: number | null = null;
     const measure = () => {
-      const next = measureComposerMultilinePrompt(body);
-      if (next !== null) setIsMultiline(next);
+      if (frame !== null) return;
+      frame = requestAnimationFrame(() => {
+        frame = null;
+        const next = measureComposerMultilinePrompt(body);
+        if (next !== null) setIsMultiline(next);
+      });
     };
     measure();
     const editor = body.querySelector<HTMLElement>('[data-testid="composer-editor"]');
     const stopObserving = observeResize(editor ? [body, editor] : body, measure);
     const mutationObserver = new MutationObserver(measure);
     mutationObserver.observe(body, { childList: true, characterData: true, subtree: true });
     return () => {
+      if (frame !== null) cancelAnimationFrame(frame);
       stopObserving();
       mutationObserver.disconnect();
     };

apps/web/src/components/sidebar/routeReveal.browser.test.tsx

@@ -17,6 +17,7 @@ function RevealList(props: {
         <li
           key={key}
           ref={key === props.activeKey ? (row) => revealActiveRow(row, key) : undefined}
+          tabIndex={-1}
           style={{ height: ROW_HEIGHT_PX }}
         >
           {key}
@@ -46,6 +47,24 @@ describe("useActiveRowReveal", () => {
     await expect.element(target).toBeInViewport();
   });
 
+  it("leaves the list where it is when the user opened the thread from its focused row", async () => {
+    const reveal = newReveal();
+    const keys = rowKeys(50);
+    await using screen = await render(
+      <RevealList reveal={reveal} rowKeys={keys} activeKey="thread-0" />,
+      // No router: the route arrives as the activeKey prop.
+      { router: false },
+    );
+    const list = screen.getByRole("list", { name: "Threads" }).element();
+    // thread-2 is half above the top edge, where a reveal would scroll it fully in.
+    list.scrollTop = 60;
+    screen.getByText("thread-2", { exact: true }).element().focus({ preventScroll: true });
+
+    await screen.rerender(<RevealList reveal={reveal} rowKeys={keys} activeKey="thread-2" />);
+
+    expect(list.scrollTop).toBe(60);
+  });
+
   it("reveals the active row when it mounts after the shells arrive", async () => {
     const reveal = newReveal();
     // The route already names the thread, but its environment's shells have not

apps/web/src/components/sidebar/routeReveal.ts

@@ -38,6 +38,9 @@ export function useActiveRowReveal(reveal: SidebarRouteReveal, routeKey: string
     if (reveal.pendingKey !== rowKey && !mountRevealOwed.current) return;
     completeSidebarRouteReveal(reveal);
     mountRevealOwed.current = false;
+    // A row that holds focus was clicked or reached by keyboard, so it is already in view;
+    // scrolling it would force a layout in the middle of the thread switch's commit.
+    if (row.contains(row.ownerDocument.activeElement)) return;
     row.scrollIntoView({ block: "nearest" });
   };
 }