fix(web): follow composer and rail layout

web/src/structured.test.ts

@@ -935,6 +935,7 @@ describe("structured connection: reconnection/error/loading", () => {
 describe("structured connection: transcript scrolling", () => {
   let frames: Map<number, FrameRequestCallback>;
   let nextFrame: number;
+  let resizeCallback: ResizeObserverCallback;
 
   beforeEach(() => {
     vi.clearAllMocks();
@@ -949,6 +950,14 @@ describe("structured connection: transcript scrolling", () => {
     vi.stubGlobal("cancelAnimationFrame", (id: number) => {
       frames.delete(id);
     });
+    vi.stubGlobal("ResizeObserver", class {
+      constructor(callback: ResizeObserverCallback) {
+        resizeCallback = callback;
+      }
+      observe(): void {}
+      unobserve(): void {}
+      disconnect(): void {}
+    });
   });
 
   afterEach(() => {
@@ -977,6 +986,13 @@ describe("structured connection: transcript scrolling", () => {
     for (const [, cb] of pending) cb(0);
   }
 
+  function resizeBody(width: number): void {
+    resizeCallback(
+      [{ contentRect: { width } } as ResizeObserverEntry],
+      {} as ResizeObserver,
+    );
+  }
+
   it("shows the bottom after the initial transcript snapshot is laid out", () => {
     const ws = new FakeSocket();
     const conn = mount({ sockets: [ws] });
@@ -1034,6 +1050,55 @@ describe("structured connection: transcript scrolling", () => {
     conn.dispose();
   });
 
+  it.each([
+    { position: "pinned", top: 800, expected: 880, manuallyScrolled: false },
+    { position: "manually scrolled", top: 180, expected: 180, manuallyScrolled: true },
+  ])("preserves the $position position when the composer grows", ({ top, expected, manuallyScrolled }) => {
+    const ws = new FakeSocket();
+    const conn = mount({ sockets: [ws] });
+    const { scroll, values } = metrics(conn, 1000, 200);
+    ws.open();
+    ws.emit({ type: "transcript_snapshot", snapshot: { cursor: 0, rows: [] } });
+    runFrames();
+
+    values.top = top;
+    if (manuallyScrolled) scroll.dispatchEvent(new Event("scroll"));
+    const input = conn.el.querySelector<HTMLTextAreaElement>(".structured-input")!;
+    Object.defineProperty(input, "scrollHeight", { configurable: true, value: 120 });
+    input.value = "one\ntwo\nthree";
+    input.dispatchEvent(new Event("input"));
+    values.height = 1080;
+    runFrames();
+
+    expect(input.style.height).toBe("120px");
+    expect(values.top).toBe(expected);
+    conn.dispose();
+  });
+
+  it.each([
+    { position: "pinned", top: 800, expected: 850, manuallyScrolled: false },
+    { position: "manually scrolled", top: 180, expected: 180, manuallyScrolled: true },
+  ])("preserves the $position position across the narrow breakpoint", ({ top, expected, manuallyScrolled }) => {
+    const ws = new FakeSocket();
+    const conn = mount({ sockets: [ws] });
+    const { scroll, values } = metrics(conn, 1000, 200);
+    ws.open();
+    ws.emit({ type: "transcript_snapshot", snapshot: { cursor: 0, rows: [] } });
+    runFrames();
+
+    values.top = top;
+    if (manuallyScrolled) scroll.dispatchEvent(new Event("scroll"));
+    resizeBody(500);
+    values.clientHeight = 150;
+    runFrames();
+
+    expect(conn.el.querySelector(".structured-body")!.classList).toContain("st-narrow");
+    expect(values.top).toBe(expected);
+    resizeBody(500);
+    expect(frames.size).toBe(0);
+    conn.dispose();
+  });
+
   it("preserves a user-scrolled position across updates, approvals, and tool expansion", () => {
     const ws = new FakeSocket();
     const conn = mount({ sockets: [ws] });
@@ -1143,10 +1208,17 @@ describe("structured connection: transcript scrolling", () => {
 
     conn.onUnmount();
     expect(frames.size).toBe(0);
+    const input = conn.el.querySelector<HTMLTextAreaElement>(".structured-input")!;
+    Object.defineProperty(input, "scrollHeight", { configurable: true, value: 120 });
+    input.dispatchEvent(new Event("input"));
+    resizeBody(500);
+    expect(frames.size).toBe(0);
     conn.onMount({ focus: false });
     expect(frames.size).toBe(2);
     conn.dispose();
     expect(frames.size).toBe(0);
+    resizeBody(700);
+    expect(conn.el.querySelector(".structured-body")!.classList).toContain("st-narrow");
     runFrames();
     expect(values.top).toBe(25);
   });