fix(app): keep chat reading position stable while scrolling (#5945)

* fix(app): keep chat reading position stable while scrolling Prepend restoration froze the reading offset while wheel input and delayed image measurements changed layout. Give the viewport one reading anchor, preserve concurrent user movement, and apply virtual row measurements before paint. Suspend automatic pagination while explicit message navigation settles. * fix(app): type the reading anchor scroll intent explicitly * fix(app): retain the reading anchor through virtualization startup Keep the reader pinned while the first virtual range mounts. Strengthen the scroll regression detector to compare shared rows when the original anchor unmounts, and consume fixture replies only when they are emitted. * test(app): distinguish delayed wheel input from idle scroll drift CI frames can arrive long after their wheel events. Start idle assertions after the input driver finishes, and account for an image entered by wheel input expanding below the new reading line. Replayed all six CI captures without false positives while retaining the original 76 baseline jumps. * test(app): bound image growth allowance to displaced reading rows Require the entered image top to follow wheel input before attributing movement of a later row to its growth. Calibrate the detector against simultaneous viewport jumps and jumps of the growing row itself. * test(app): clamp scroll expectations at the history boundary
2026-11-11 02:46:29 +00:00 · 2026-10-03 10:12:01 +01:00
parent 5293ddac3f
commit 8aec0787d2
9 changed files with 773 additions and 224 deletions
@@ -0,0 +1,81 @@
+import { test } from "../support/fixtures";
+import { expect } from "@playwright/test";
+import {
+  findScrollJumps,
+  type ScrollFrame,
+  observeTimelinePages,
+  openOnlyTimelineTail,
+  recordUpwardTraversal,
+  reportScrollJumps,
+  scrollCadences,
+  withVariedTimeline,
+} from "../support/helpers/timeline-scroll-smoothness";
+
+test("scroll detector distinguishes entered image growth from a simultaneous viewport jump", () => {
+  const before: ScrollFrame = {
+    at: 1000,
+    scrollTop: 1000,
+    scrollHeight: 5000,
+    viewportHeight: 800,
+    virtualized: true,
+    loading: false,
+    rows: [
+      { id: "image", top: -150, height: 150 },
+      { id: "reading", top: 0, height: 100 },
+    ],
+    anchor: "reading",
+    wheelTotal: 0,
+    lastWheelAt: 0,
+    inputFinishedAt: null,
+    imageLoads: 0,
+    mounted: 0,
+    unmounted: 0,
+  };
+  const after: ScrollFrame = {
+    ...before,
+    at: 1400,
+    wheelTotal: 100,
+    lastWheelAt: 1050,
+    rows: [
+      { id: "image", top: -50, height: 1150 },
+      { id: "reading", top: 1100, height: 100 },
+    ],
+  };
+  expect(findScrollJumps([before, after])).toEqual([]);
+  expect(
+    findScrollJumps([
+      { ...before, scrollTop: 200 },
+      {
+        ...after,
+        wheelTotal: 1000,
+        rows: after.rows.map((row) => ({ ...row, top: row.top + 100 })),
+      },
+    ]),
+  ).toEqual([]);
+  expect(
+    findScrollJumps([
+      before,
+      { ...after, rows: after.rows.map((row) => ({ ...row, top: row.top - 500 })) },
+    ]),
+  ).toHaveLength(1);
+  expect(
+    findScrollJumps([
+      { ...before, anchor: "image", rows: [{ id: "image", top: 0, height: 150 }] },
+      { ...after, rows: [{ id: "image", top: 600, height: 1150 }] },
+    ]),
+  ).toHaveLength(1);
+});
+
+for (const cadence of scrollCadences) {
+  test(`varied timeline preserves reading position during ${cadence.name} upward scrolling`, async ({
+    page,
+  }, testInfo) => {
+    test.setTimeout(180_000);
+    await withVariedTimeline(async (agent, newestPrompt) => {
+      const pages = observeTimelinePages(page, agent.agentId);
+      await openOnlyTimelineTail(page, agent, newestPrompt, pages);
+      const frames = await recordUpwardTraversal(page, cadence, testInfo);
+      await reportScrollJumps(page, testInfo, frames, pages);
+    });
+  });
+}
@@ -0,0 +1,430 @@
+import { writeFile } from "node:fs/promises";
+import { createServer } from "node:http";
+import { expect, type Page, type TestInfo } from "@playwright/test";
+import { seedMockAgentWorkspace, type MockAgentWorkspace } from "./mock-agent";
+import { openAgentTimeline, expectTimelinePromptVisible } from "./timeline-pagination";
+
+export const scrollCadences = [
+  { name: "slow", delta: 160, intervalMs: 50, steps: 1000 },
+  { name: "medium", delta: 480, intervalMs: 32, steps: 400 },
+  { name: "fast", delta: 1200, intervalMs: 16, steps: 180 },
+] as const;
+
+type Cadence = (typeof scrollCadences)[number];
+interface RowFrame {
+  id: string;
+  top: number;
+  height: number;
+}
+export interface ScrollFrame {
+  at: number;
+  scrollTop: number;
+  scrollHeight: number;
+  viewportHeight: number;
+  virtualized: boolean;
+  loading: boolean;
+  rows: RowFrame[];
+  anchor: string | null;
+  wheelTotal: number;
+  lastWheelAt: number;
+  inputFinishedAt: number | null;
+  imageLoads: number;
+  mounted: number;
+  unmounted: number;
+}
+interface Recording {
+  frames: ScrollFrame[];
+  finishInput(): void;
+  stop(): void;
+}
+interface TimelinePage {
+  direction: string;
+  count: number;
+  hasOlder: boolean;
+  at: number;
+}
+
+/** Real HTTP image responses, with intrinsic dimensions unavailable until the response arrives. */
+export async function withVariedTimeline(
+  run: (agent: MockAgentWorkspace, newestPrompt: string) => Promise<void>,
+): Promise<void> {
+  const timers = new Set<ReturnType<typeof setTimeout>>();
+  const server = createServer((request, response) => {
+    const portrait = request.url?.includes("portrait");
+    const timer = setTimeout(
+      () => {
+        timers.delete(timer);
+        response.writeHead(200, {
+          "Content-Type": "image/svg+xml",
+          "Cache-Control": "no-store",
+          "Access-Control-Allow-Origin": "*",
+        });
+        response.end(
+          `<svg xmlns="http://www.w3.org/2000/svg" width="${portrait ? 320 : 960}" height="${portrait ? 640 : 240}"><rect width="100%" height="100%" fill="${portrait ? "#936" : "#369"}"/><text x="20" y="50" fill="white" font-size="24">Delayed ${portrait ? "portrait" : "landscape"}</text></svg>`,
+        );
+      },
+      portrait ? 900 : 350,
+    );
+    timers.add(timer);
+  });
+  await new Promise<void>((resolve) => server.listen(0, "127.0.0.1", resolve));
+  const address = server.address();
+  if (!address || typeof address === "string") throw new Error("Image server did not listen");
+  const imageUrl = `http://127.0.0.1:${address.port}`;
+  let agent: MockAgentWorkspace | undefined;
+  try {
+    agent = await seedMockAgentWorkspace({
+      repoPrefix: "timeline-scroll-smoothness-",
+      title: "Varied timeline scroll reproduction",
+      featureValues: {
+        mockAssistantResponses: Array.from({ length: 70 }, (_, turn) =>
+          [
+            `## Review of turn ${turn}`,
+            "A short response followed by blocks with different heights. Read the comparison, inspect the screenshot, and continue to the next turn.",
+            "| Component | Observation | Next step |\n| --- | --- | --- |\n| Pagination | Older turns arrive together | Keep the visible text anchored |\n| Images | Intrinsic sizes arrive later | Preserve the reading position |\n| Virtualization | Estimated heights become measured heights | Keep wheel input moving upward |",
+            `![Delayed landscape](${imageUrl}/${turn}/landscape.svg)`,
+            "```typescript\nconst viewport = { top: 0, height: 800 };\nfor (const row of history) {\n  inspect(row, viewport);\n}\n```",
+            "> Keep reading while content above the viewport changes.",
+            "- Short item\n- A longer item that wraps when the available width is smaller and adds another height variation\n- Final item",
+            `![Delayed portrait](${imageUrl}/${turn}/portrait.svg)`,
+            "Review complete. The next prompt starts a separate turn.",
+          ].join("\n\n"),
+        ),
+      },
+    });
+    for (let turn = 0; turn < 70; turn += 1) {
+      await agent.client.sendAgentMessage(
+        agent.agentId,
+        `Review turn ${turn}. ${"Compare the image and table with the earlier result. ".repeat(1 + (turn % 4))}`,
+      );
+      await agent.client.waitForFinish(agent.agentId, 15_000);
+    }
+    await run(
+      agent,
+      `Review turn 69. ${"Compare the image and table with the earlier result. ".repeat(2)}`,
+    );
+  } finally {
+    await agent?.cleanup();
+    for (const timer of timers) clearTimeout(timer);
+    server.closeAllConnections();
+    await new Promise<void>((resolve) => server.close(() => resolve()));
+  }
+}
+
+export function observeTimelinePages(page: Page, agentId: string): TimelinePage[] {
+  const pages: TimelinePage[] = [];
+  page.on("websocket", (socket) => {
+    socket.on("framereceived", ({ payload }) => {
+      const envelope = JSON.parse(String(payload));
+      const message = envelope.message;
+      if (message?.type !== "fetch_agent_timeline_response" || message.payload.agentId !== agentId)
+        return;
+      pages.push({
+        direction: message.payload.direction,
+        count: message.payload.entries.length,
+        hasOlder: message.payload.hasOlder,
+        at: Date.now(),
+      });
+    });
+  });
+  return pages;
+}
+
+export async function openOnlyTimelineTail(
+  page: Page,
+  agent: MockAgentWorkspace,
+  newestPrompt: string,
+  pages: TimelinePage[],
+): Promise<void> {
+  await page.addInitScript(() => Reflect.set(globalThis, "__PASEO_RENDER_PROFILE_ENABLED__", true));
+  await openAgentTimeline(page, agent);
+  await expectTimelinePromptVisible(page, newestPrompt.trim());
+  await expect
+    .poll(() => pages.filter((entry) => entry.direction === "tail").length)
+    .toBeGreaterThanOrEqual(1);
+
+  expect(pages[0]).toMatchObject({ direction: "tail", hasOlder: true });
+  expect(pages[0].count).toBe(40);
+  expect(pages.filter((entry) => entry.direction === "tail")).toHaveLength(1);
+  expect(
+    pages.filter((entry) => entry.direction === "after").every((entry) => entry.count === 0),
+  ).toBe(true);
+  await expect(page.getByText(/^Review turn 0\./)).toHaveCount(0);
+  // Start each cadence after the tail's initial image loads and bottom anchoring.
+  await page.waitForTimeout(1500);
+  expect(pages.filter((entry) => entry.direction === "before")).toHaveLength(0);
+}
+
+export async function recordUpwardTraversal(
+  page: Page,
+  cadence: Cadence,
+  testInfo: TestInfo,
+): Promise<ScrollFrame[]> {
+  const stopTrace = await traceScrollIfRequested(page, testInfo, cadence);
+  const timeline = page.getByTestId("agent-chat-scroll");
+  await timeline.hover();
+  await timeline.evaluate((viewport) => {
+    const scroll = viewport as HTMLElement;
+    const frames: ScrollFrame[] = [];
+    let frame = 0;
+    let wheelTotal = 0;
+    let lastWheelAt = -Infinity;
+    let inputFinishedAt: number | null = null;
+    let imageLoads = 0;
+    let mounted = 0;
+    let unmounted = 0;
+    let previousIds = new Set<string>();
+    const wheel = (event: WheelEvent) => {
+      wheelTotal += -event.deltaY;
+      lastWheelAt = performance.now();
+    };
+    const imageLoad = (event: Event) => {
+      if (event.target instanceof HTMLImageElement) imageLoads += 1;
+    };
+    scroll.addEventListener("wheel", wheel, { passive: true });
+    scroll.addEventListener("load", imageLoad, true);
+    const observed = new Set<Element>();
+    const capture = (replaceLast: boolean) => {
+      const rect = scroll.getBoundingClientRect();
+      const elements = Array.from(scroll.querySelectorAll<HTMLElement>("[data-history-row-id]"));
+      const current = new Set<Element>([scroll, ...elements]);
+      if (scroll.firstElementChild) current.add(scroll.firstElementChild);
+      for (const element of observed) {
+        if (!current.has(element)) {
+          resizeObserver.unobserve(element);
+          observed.delete(element);
+        }
+      }
+      for (const element of current) {
+        if (!observed.has(element)) {
+          resizeObserver.observe(element);
+          observed.add(element);
+        }
+      }
+      const rows = elements.map((row) => {
+        const box = row.getBoundingClientRect();
+        return { id: row.dataset.historyRowId!, top: box.top - rect.top, height: box.height };
+      });
+      const ids = new Set(rows.map((row) => row.id));
+      mounted += [...ids].filter((id) => !previousIds.has(id)).length;
+      unmounted += [...previousIds].filter((id) => !ids.has(id)).length;
+      previousIds = ids;
+      // Preserve the row crossing the reading line. Rows below an expanding
+      // visible image legitimately move even when the reading position is stable.
+      const anchor = rows.find((row) => row.top < rect.height && row.top + row.height > 8);
+      const snapshot: ScrollFrame = {
+        at: performance.now(),
+        scrollTop: scroll.scrollTop,
+        scrollHeight: scroll.scrollHeight,
+        viewportHeight: rect.height,
+        rows,
+        anchor: anchor?.id ?? null,
+        virtualized: scroll.id.includes("virtualized"),
+        loading: !!scroll.querySelector('[data-testid="load-older-history-spinner"]'),
+        wheelTotal,
+        lastWheelAt,
+        inputFinishedAt,
+        imageLoads,
+        mounted,
+        unmounted,
+      };
+      if (replaceLast && frames.length > 0) frames[frames.length - 1] = snapshot;
+      else frames.push(snapshot);
+    };
+    // rAF precedes ResizeObserver. Refresh that frame after resize delivery so
+    // it represents the final layout, not geometry repaired before the paint.
+    const resizeObserver = new ResizeObserver(() => capture(true));
+    const sample = () => {
+      capture(false);
+      frame = requestAnimationFrame(sample);
+    };
+    const recording: Recording = {
+      frames,
+      finishInput() {
+        inputFinishedAt = performance.now();
+      },
+      stop() {
+        cancelAnimationFrame(frame);
+        resizeObserver.disconnect();
+        scroll.removeEventListener("wheel", wheel);
+        scroll.removeEventListener("load", imageLoad, true);
+      },
+    };
+    Reflect.set(window, "__timelineScrollRecording", recording);
+    const reset = Reflect.get(window, "__PASEO_RESET_RENDER_PROFILE__");
+    if (typeof reset === "function") reset();
+    sample();
+  });
+  // No settling between inputs: exercise wheel input overlapping prepend and measurement.
+  for (let step = 0; step < cadence.steps; step += 1) {
+    await page.mouse.wheel(0, -cadence.delta);
+    await page.waitForTimeout(cadence.intervalMs);
+  }
+  await page.evaluate(() => {
+    (Reflect.get(window, "__timelineScrollRecording") as Recording).finishInput();
+  });
+  await page.waitForTimeout(1500);
+  await stopTrace();
+  return page.evaluate(() => {
+    const recording = Reflect.get(window, "__timelineScrollRecording") as Recording;
+    recording.stop();
+    return recording.frames;
+  });
+}
+
+export function findScrollJumps(frames: ScrollFrame[]) {
+  return frames.flatMap((current, index) => {
+    const previous = frames[index - 1];
+    if (!previous?.anchor) return [];
+    // Wheel input can move the reading line onto an image before it expands.
+    // Follow that intended row, not text now below the image.
+    const inputDelta = current.wheelTotal - previous.wheelTotal;
+    const availableScroll = Math.min(inputDelta, previous.scrollTop);
+    const readingLine = 8 - availableScroll;
+    const currentRows = new Map(current.rows.map((row) => [row.id, row]));
+    const intendedRow = previous.rows.find((row) => row.top + row.height > readingLine);
+    const intendedHeight = intendedRow && currentRows.get(intendedRow.id)?.height;
+    const anchor = previous.anchor;
+    const before = currentRows.has(anchor)
+      ? previous.rows.find((row) => row.id === anchor)!
+      : previous.rows
+          .filter((row) => currentRows.has(row.id))
+          .sort((left, right) => Math.abs(left.top - 8) - Math.abs(right.top - 8))[0];
+    if (!before) {
+      throw new Error(`No shared reading geometry between frames ${index - 1} and ${index}`);
+    }
+    const after = currentRows.get(before.id)!;
+    const movement = after.top - before.top;
+    // A busy main thread can deliver wheel movement hundreds of milliseconds
+    // after its event. Only assert idle stability after the driver stops input.
+    const idle = current.inputFinishedAt !== null && previous.at - current.inputFinishedAt > 250;
+    const recent = frames.findLast((frame) => frame.at <= previous.at - 100);
+    const wheelBudget = current.wheelTotal - (recent?.wheelTotal ?? 0);
+    // Growth displaces only later rows. It never grants extra movement to the
+    // image itself, and its top must still follow the input budget.
+    const enteredRowGrowth =
+      inputDelta > 0 &&
+      intendedRow &&
+      intendedHeight !== undefined &&
+      before.top >= intendedRow.top + intendedRow.height &&
+      Math.abs(currentRows.get(intendedRow.id)!.top - intendedRow.top - availableScroll) <= 32
+        ? Math.max(0, intendedHeight - intendedRow.height)
+        : 0;
+    const excessForward = movement > wheelBudget + enteredRowGrowth + 32;
+    // Upward wheel input moves the same text DOWN the viewport. A negative move
+    // is a reversal, independent of legitimate scrollTop compensation on prepend.
+    if (movement >= -8 && (!idle || Math.abs(movement) <= 8) && !excessForward) return [];
+    return [
+      {
+        frame: index,
+        at: current.at,
+        row: before.id,
+        movement,
+        idle,
+        excessForward,
+        resizedRows: current.rows.flatMap((row) => {
+          const old = previous.rows.find((candidate) => candidate.id === row.id);
+          return old && old.height !== row.height
+            ? [{ id: row.id, before: old.height, after: row.height }]
+            : [];
+        }),
+        loading: current.loading || previous.loading,
+        virtualized: current.virtualized,
+        heightChange: current.scrollHeight - previous.scrollHeight,
+        imageLoads: current.imageLoads - previous.imageLoads,
+        frameGapMs: current.at - previous.at,
+      },
+    ];
+  });
+}
+
+export async function reportScrollJumps(
+  page: Page,
+  testInfo: TestInfo,
+  frames: ScrollFrame[],
+  pages: TimelinePage[],
+): Promise<void> {
+  const jumps = findScrollJumps(frames);
+  const react = await page.evaluate(() => Reflect.get(window, "__PASEO_RENDER_PROFILE__") ?? []);
+  const gaps = frames
+    .slice(1)
+    .map((frame, index) => frame.at - frames[index].at)
+    .sort((a, b) => a - b);
+  const summary = {
+    browserTimeOrigin: await page.evaluate(() => performance.timeOrigin),
+    frames: frames.length,
+    pages,
+    jumpCount: jumps.length,
+    worstReversePx: Math.max(0, ...jumps.map((jump) => -jump.movement)),
+    frameGapP95Ms: gaps[Math.floor(gaps.length * 0.95)],
+    frameGapMaxMs: gaps.at(-1),
+    imageLoads: frames.at(-1)?.imageLoads,
+    mounted: frames.at(-1)?.mounted,
+    unmounted: frames.at(-1)?.unmounted,
+    jumps,
+  };
+  await writeFile(testInfo.outputPath("scroll-summary.json"), JSON.stringify(summary, null, 2));
+  await writeFile(testInfo.outputPath("scroll-frames.json"), JSON.stringify(frames));
+  await writeFile(testInfo.outputPath("scroll-react-profile.json"), JSON.stringify(react));
+  await testInfo.attach("scroll-summary", {
+    body: JSON.stringify(summary, null, 2),
+    contentType: "application/json",
+  });
+  await testInfo.attach("scroll-frames", {
+    body: JSON.stringify(frames),
+    contentType: "application/json",
+  });
+  await testInfo.attach("scroll-react-profile", {
+    body: JSON.stringify(react),
+    contentType: "application/json",
+  });
+  await testInfo.attach("scroll-end", { body: await page.screenshot(), contentType: "image/png" });
+  console.log(
+    `[scroll] ${testInfo.title}: ${JSON.stringify({ ...summary, jumps: jumps.slice(0, 5) })}`,
+  );
+  expect(
+    pages.filter((entry) => entry.direction === "before").length,
+    "must exercise multiple older pages",
+  ).toBeGreaterThanOrEqual(2);
+  expect(
+    frames.some((frame) => frame.virtualized),
+    "must exercise virtualization",
+  ).toBe(true);
+  expect(summary.imageLoads, "must exercise image completion during traversal").toBeGreaterThan(0);
+  expect(
+    jumps,
+    "visible text must not reverse direction while scrolling upward or move after input stops",
+  ).toEqual([]);
+}
+
+async function traceScrollIfRequested(
+  page: Page,
+  testInfo: TestInfo,
+  cadence: Cadence,
+): Promise<() => Promise<void>> {
+  if (process.env.PASEO_TIMELINE_SCROLL_TRACE !== cadence.name) return async () => {};
+  const cdp = await page.context().newCDPSession(page);
+  await cdp.send("Tracing.start", {
+    categories:
+      "devtools.timeline,v8.execute,blink.user_timing,disabled-by-default-v8.cpu_profiler",
+    transferMode: "ReturnAsStream",
+  });
+  return async () => {
+    const completed = new Promise<string>((resolve) => {
+      cdp.once("Tracing.tracingComplete", ({ stream }) => resolve(stream!));
+    });
+    await cdp.send("Tracing.end");
+    const stream = await completed;
+    const chunks: string[] = [];
+    for (;;) {
+      const chunk = await cdp.send("IO.read", { handle: stream });
+      chunks.push(chunk.base64Encoded ? Buffer.from(chunk.data, "base64").toString() : chunk.data);
+      if (chunk.eof) break;
+    }
+    await cdp.send("IO.close", { handle: stream });
+    await cdp.detach();
+    const path = testInfo.outputPath("scroll-chromium-trace.json");
+    await writeFile(path, chunks.join(""));
+    await testInfo.attach("scroll-chromium-trace", { path, contentType: "application/json" });
+  };
+}