Make line wrapping responsive in large diffs (#8006)

* perf(app): make diff line wrapping responsive on large changes Wrapping eagerly shaped every expanded line before the viewport could paint. Estimate off-screen heights and measure rows as they enter the window, preserving the scroll anchor as geometry becomes exact. Add shared layout and browser benchmarks to track toggle latency and shaping cost. * fix(app): preserve wrapped diff anchors across bounded relayouts Resizing can move the viewed row outside the old materialization window. Keep its estimated wrap geometry available to anchor resolution, and measure the anchored row in the same layout pass so inaccurate estimates cannot discard the exact fragment or pixel offset.
2026-10-27 11:42:52 +00:00 · 2026-10-27 16:09:53 +07:00
parent ef58ace424
commit 50623965b4
11 changed files with 872 additions and 29 deletions
@@ -273,6 +273,26 @@ Playwright dispatch lateness. Set `PASEO_PROFILE_TYPING_KEYS`, `PASEO_PROFILE_TY
 `PASEO_PROFILE_CPU_PATH` and `PASEO_PROFILE_TRACE_PATH` captures run separately after the latency
 measurements so profiling overhead does not contaminate them.

+For diff layout, two benchmarks share one corpus
+(`packages/app/src/git/diff-document/benchmark-corpus.ts`: a real source file, a changelog with
+600-character lines, generated code with ligature punctuation, and JSON). The node bench runs the
+shared document model with the web and native measurers replaced by counters, so it reports how
+many strings each backend would have to shape, which is the cost that survives the move to canvas
+or Skia:
+
+```bash
+cd packages/app && npx vitest bench --project unit src/git/diff-document/model.bench.ts
+```
+
+The browser benchmark opens the same corpus as a working diff, toggles line wrap from the toolbar,
+and reports pointer-down to painted latency, the longest main-thread task, and the `DiffDocument`
+React commit time for cold and warm toggles, then scroll frame times in both modes:
+
+```bash
+cd packages/app && PASEO_DIFF_PERF_E2E=1 npx playwright test --project=browser \
+  e2e/browser/diff-performance.spec.ts -g "toggling line wrap"
+```
+
 Set `PASEO_PROFILE_TYPING_SCENARIO=height-growth` to alternate `Shift+Enter` and printable input.
 That report includes input and composer height changes plus React work grouped into composer,
 stream, and ancestor/root scopes. Ancestor/root timings include descendant work because the Profiler
@@ -1,6 +1,7 @@
 import { writeFile } from "node:fs/promises";
 import path from "node:path";
 import type { CDPSession, Locator, Page, TestInfo } from "@playwright/test";
+import { pullRequestCorpus } from "../../src/git/diff-document/benchmark-corpus";
 import { buildHostWorkspaceRoute } from "../../src/utils/host-routes";
 import { test, expect } from "../support/fixtures";
 import { getServerId } from "../support/helpers/server-id";
@@ -296,8 +297,187 @@ diffPerfDescribe("Diff canvas performance", () => {
      await repo.cleanup().catch(() => undefined);
    }
  });
+  test("toggling line wrap on a pull-request-sized diff", async ({ page }, testInfo) => {
+    test.setTimeout(300_000);
+    const corpus = await createPullRequestCorpusRepository();
+    const client = await connectSeedClient();
+
+    try {
+      const created = await client.createWorkspace({
+        source: { kind: "directory", path: corpus.repo.path },
+      });
+      if (!created.workspace) {
+        throw new Error(created.error ?? "Failed to create wrap performance workspace");
+      }
+
+      await configureUnwrappedUnifiedDiff(page);
+      await instrumentDiffLatency(page);
+      await page.setViewportSize({ width: 1400, height: 900 });
+      await page.goto(buildHostWorkspaceRoute(getServerId(), created.workspace.id));
+      await waitForWorkspaceTabsVisible(page);
+      const openStartedAt = performance.now();
+      await openChangesPanel(page, 90_000);
+      await expect(page.getByTestId("git-diff-canvas")).toBeVisible({ timeout: 30_000 });
+      const open = {
+        readyMs: Math.round(performance.now() - openStartedAt),
+        ...(await readDiffDocumentRenders(page)),
+      };
+
+      const toggles: WrapToggleMeasurement[] = [];
+      for (const name of [
+        "wrap on (cold)",
+        "wrap off (cold)",
+        "wrap on (warm)",
+        "wrap off (warm)",
+      ]) {
+        toggles.push(await measureWrapToggle(page, name));
+      }
+      const unwrappedScroll = await continuouslyScrollCanvas(page);
+      toggles.push(await measureWrapToggle(page, "wrap on (warm, scrolled)"));
+      const wrappedScroll = await continuouslyScrollCanvas(page);
+
+      const report = {
+        scenario: "wrap-toggle",
+        sourceFiles: corpus.fileCount,
+        sourceLines: corpus.lineCount,
+        open,
+        toggles,
+        scroll: {
+          unwrapped: summarizeScroll(unwrappedScroll),
+          wrapped: summarizeScroll(wrappedScroll),
+        },
+      };
+      await attachPerformanceReport(testInfo, report);
+      console.log(`[perf] Wrap toggle: ${JSON.stringify(report)}`);
+
+      const [wrapOn, wrapOff] = toggles;
+      expect(wrapOn!.scrollHeightAfter).toBeGreaterThan(wrapOn!.scrollHeightBefore);
+      expect(wrapOff!.scrollHeightAfter).toBe(wrapOn!.scrollHeightBefore);
+    } finally {
+      await client.close().catch(() => undefined);
+      await corpus.repo.cleanup().catch(() => undefined);
+    }
+  });
 });

+interface WrapToggleMeasurement {
+  name: string;
+  /** Pointer down on the toolbar button to two painted frames after the document resized. */
+  latencyMs: number;
+  scrollHeightBefore: number;
+  scrollHeightAfter: number;
+  longestTaskMs: number;
+  longTaskTotalMs: number;
+  diffDocumentCommits: number;
+  diffDocumentRenderMs: number;
+}
+
+interface LongTask {
+  startTime: number;
+  duration: number;
+}
+
+interface DiffLatencyScope {
+  __PASEO_RENDER_PROFILE_ENABLED__?: boolean;
+  __PASEO_RENDER_PROFILE__?: Array<{ id: string; actualDuration: number }>;
+  __PASEO_RESET_RENDER_PROFILE__?: () => void;
+  __PASEO_LONG_TASKS__?: LongTask[];
+  __PASEO_WRAP_TOGGLE_AT__?: number;
+}
+
+async function createPullRequestCorpusRepository() {
+  const files = pullRequestCorpus();
+  const repo = await createTempGitRepo("diff-wrap-perf-", {
+    files: files.map((file) => ({ path: file.path, content: "baseline\n" })),
+  });
+  for (const file of files) {
+    await writeFile(path.join(repo.path, file.path), `${file.lines.join("\n")}\n`);
+  }
+  return {
+    repo,
+    fileCount: files.length,
+    lineCount: files.reduce((count, file) => count + file.lines.length, 0),
+  };
+}
+
+async function instrumentDiffLatency(page: Page): Promise<void> {
+  await page.addInitScript(() => {
+    const scope = globalThis as typeof globalThis & DiffLatencyScope;
+    scope.__PASEO_RENDER_PROFILE_ENABLED__ = true;
+    scope.__PASEO_LONG_TASKS__ = [];
+    new PerformanceObserver((list) => {
+      for (const entry of list.getEntries()) {
+        scope.__PASEO_LONG_TASKS__?.push({ startTime: entry.startTime, duration: entry.duration });
+      }
+    }).observe({ type: "longtask", buffered: true });
+  });
+}
+
+async function readDiffDocumentRenders(
+  page: Page,
+): Promise<{ diffDocumentCommits: number; diffDocumentRenderMs: number }> {
+  return page.evaluate(() => {
+    const scope = globalThis as typeof globalThis & DiffLatencyScope;
+    const samples = (scope.__PASEO_RENDER_PROFILE__ ?? []).filter(
+      (sample) => sample.id === "DiffDocument",
+    );
+    return {
+      diffDocumentCommits: samples.length,
+      diffDocumentRenderMs: Math.round(
+        samples.reduce((total, sample) => total + sample.actualDuration, 0),
+      ),
+    };
+  });
+}
+
+async function measureWrapToggle(page: Page, name: string): Promise<WrapToggleMeasurement> {
+  const scroll = page.getByTestId("git-diff-scroll");
+  const scrollHeightBefore = await scroll.evaluate((element) => element.scrollHeight);
+  await page.evaluate(() => {
+    const scope = globalThis as typeof globalThis & DiffLatencyScope;
+    scope.__PASEO_RESET_RENDER_PROFILE__?.();
+    scope.__PASEO_LONG_TASKS__ = [];
+    scope.__PASEO_WRAP_TOGGLE_AT__ = undefined;
+    document.addEventListener(
+      "pointerdown",
+      () => {
+        scope.__PASEO_WRAP_TOGGLE_AT__ = performance.now();
+      },
+      { capture: true, once: true },
+    );
+  });
+  await page.getByTestId("changes-toggle-wrap-lines").click();
+  const measured = await scroll.evaluate(async (element, before) => {
+    const scope = globalThis as typeof globalThis & DiffLatencyScope;
+    const deadline = performance.now() + 240_000;
+    while (element.scrollHeight === before && performance.now() < deadline) {
+      await new Promise((resolve) => setTimeout(resolve, 5));
+    }
+    await new Promise<void>((resolve) =>
+      requestAnimationFrame(() => requestAnimationFrame(() => resolve())),
+    );
+    const doneAt = performance.now();
+    // Long task entries are delivered after the task ends; give the observer a turn.
+    await new Promise((resolve) => setTimeout(resolve, 50));
+    const startedAt = scope.__PASEO_WRAP_TOGGLE_AT__ ?? doneAt;
+    const longTasks = (scope.__PASEO_LONG_TASKS__ ?? []).filter(
+      (task) => task.startTime + task.duration >= startedAt && task.startTime <= doneAt,
+    );
+    return {
+      latencyMs: Math.round(doneAt - startedAt),
+      scrollHeightAfter: element.scrollHeight,
+      longestTaskMs: Math.round(Math.max(0, ...longTasks.map((task) => task.duration))),
+      longTaskTotalMs: Math.round(longTasks.reduce((total, task) => total + task.duration, 0)),
+    };
+  }, scrollHeightBefore);
+  return { name, scrollHeightBefore, ...measured, ...(await readDiffDocumentRenders(page)) };
+}
+
+function summarizeScroll(report: Awaited<ReturnType<typeof continuouslyScrollCanvas>>) {
+  const { frameSignatures: _frameSignatures, ...summary } = report;
+  return summary;
+}
+
 async function measureAdditionalPerformanceScenario(
  page: Page,
  cdp: CDPSession,
@@ -0,0 +1,65 @@
+import { readFileSync } from "node:fs";
+import path from "node:path";
+
+/**
+ * The diff shapes the layout benchmarks measure, shared by the node bench
+ * (`model.bench.ts`) and the browser benchmark (`e2e/browser/diff-performance.spec.ts`)
+ * so both report against the same text.
+ */
+export interface BenchmarkCorpusFile {
+  path: string;
+  lines: string[];
+}
+
+function repeatToLength(unit: string, length: number): string {
+  return unit.repeat(Math.ceil(length / unit.length)).slice(0, length);
+}
+
+/** A real source file: short lines, a few with ligature punctuation. */
+export function realCodeCorpusFile(): BenchmarkCorpusFile {
+  const source = readFileSync(path.resolve(__dirname, "../diff-pane.tsx"), "utf8");
+  return { path: "packages/app/src/git/diff-pane.tsx", lines: source.split("\n") };
+}
+
+/** The CHANGELOG shape that once froze the renderer: 42 lines, longest ~600 characters. */
+export function changelogCorpusFile(): BenchmarkCorpusFile {
+  const sentence =
+    "- Fixed an issue where the diff viewer re-measured every line -> on toggle, which made wrapping painfully slow. ";
+  return {
+    path: "CHANGELOG.md",
+    lines: Array.from({ length: 42 }, (_, index) =>
+      repeatToLength(sentence, 200 + ((index * 97) % 398)),
+    ),
+  };
+}
+
+/** Generated TypeScript: ligature punctuation on every line, every tenth line long. */
+export function generatedCodeCorpusFile(lineCount: number): BenchmarkCorpusFile {
+  return {
+    path: "src/generated.ts",
+    lines: Array.from({ length: lineCount }, (_, index) => {
+      const base = `const handler${index} = async (event) => items.filter((item) => item.id !== ${index} && item.kind === "selected");`;
+      return index % 10 === 9 ? repeatToLength(`${base} // `, 320) : base;
+    }),
+  };
+}
+
+/** Serialized data: no ligatures, every line wraps several times. */
+export function jsonCorpusFile(lineCount: number): BenchmarkCorpusFile {
+  return {
+    path: "fixtures/data.json",
+    lines: Array.from({ length: lineCount }, (_, index) =>
+      repeatToLength(`{"id": ${index}, "label": "row ${index}", "tags": ["alpha", "beta"]}, `, 300),
+    ),
+  };
+}
+
+/** A pull-request-sized working diff: one of each shape above. */
+export function pullRequestCorpus(): BenchmarkCorpusFile[] {
+  return [
+    realCodeCorpusFile(),
+    changelogCorpusFile(),
+    generatedCodeCorpusFile(1200),
+    jsonCorpusFile(300),
+  ];
+}
@@ -0,0 +1,317 @@
+import type { ParsedDiffFile } from "@getpaseo/protocol/messages";
+import { bench, describe } from "vitest";
+import {
+  changelogCorpusFile,
+  generatedCodeCorpusFile,
+  jsonCorpusFile,
+  pullRequestCorpus,
+  realCodeCorpusFile,
+  type BenchmarkCorpusFile,
+} from "./benchmark-corpus";
+import { buildDiffDocumentModel } from "./model";
+import {
+  createAdditiveAdvances,
+  createChunkedAdvanceMeasurer,
+  createChunkedWidthMeasurer,
+  createMeasuredAdvances,
+  requiresShaping,
+} from "./text-measurement";
+import type { BuildDiffDocumentModelInput, TextMeasurer } from "./types";
+
+// The pure layout work the diff surface does when a diff opens or line wrap is
+// toggled, without a browser or Skia. Counts are the backend-independent cost
+// owners (how many strings a shaper has to see), timings are the JS overhead
+// around them:
+//   npx vitest bench --project unit src/git/diff-document/model.bench.ts
+
+const DESKTOP_VIEWPORT = { width: 1400, height: 900 };
+const PHONE_VIEWPORT = { width: 390, height: 800 };
+const CELL_WIDTH = 7.8;
+
+interface MeasurementStats {
+  /** Whole strings handed to the backend. */
+  calls: number;
+  /** Characters inside those strings; canvas and Skia cost scales with this. */
+  characters: number;
+}
+
+interface Backend {
+  measurer: TextMeasurer;
+  stats: MeasurementStats;
+}
+
+function monospaceWidth(text: string): number {
+  let width = 0;
+  for (const character of text) width += character.charCodeAt(0) > 0x2e7f ? 2 : 1;
+  return width * CELL_WIDTH;
+}
+
+function countingMeasure(stats: MeasurementStats): (text: string) => number {
+  return (text) => {
+    stats.calls += 1;
+    stats.characters += text.length;
+    return monospaceWidth(text);
+  };
+}
+
+/** Previous web backend, retained to isolate the cost of width-only probes. */
+function webBackendWithoutWidthProbe(): Backend {
+  const stats: MeasurementStats = { calls: 0, characters: 0 };
+  const measure = countingMeasure(stats);
+  return { stats, measurer: { measure, measureAdvances: createMeasuredAdvances(measure) } };
+}
+
+/**
+ * `createWebTextMeasurer` in surface.web.tsx, with canvas replaced by a counter.
+ * Width-only probes shape one string per chunk, like the native measurer; the
+ * widths are the same sums, in the same order, as the last cumulative advance.
+ */
+function webBackend(): Backend {
+  const stats: MeasurementStats = { calls: 0, characters: 0 };
+  const measure = countingMeasure(stats);
+  const additive = createAdditiveAdvances(measure);
+  return {
+    stats,
+    measurer: {
+      measure,
+      measureAdvances: createMeasuredAdvances(measure),
+      measureWidth: createChunkedWidthMeasurer((graphemes) => {
+        const text = graphemes.join("");
+        return requiresShaping(text) ? measure(text) : (additive(graphemes).at(-1) ?? 0);
+      }),
+    },
+  };
+}
+
+/** `createNativeTextMeasurer` in text.native.ts, with Skia replaced by a counter. */
+function nativeBackend(): Backend {
+  const stats: MeasurementStats = { calls: 0, characters: 0 };
+  const widths = new Map<string, number>();
+  const additive = (graphemes: readonly string[]) => {
+    let advance = 0;
+    return graphemes.map((grapheme) => {
+      let width = widths.get(grapheme);
+      if (width === undefined) {
+        stats.calls += 1;
+        stats.characters += grapheme.length;
+        width = monospaceWidth(grapheme);
+        widths.set(grapheme, width);
+      }
+      return (advance += width);
+    });
+  };
+  const shape = countingMeasure(stats);
+  return {
+    stats,
+    measurer: {
+      measure(text) {
+        return requiresShaping(text) ? shape(text) : (additive([...text]).at(-1) ?? 0);
+      },
+      measureWidth: createChunkedWidthMeasurer((graphemes) => {
+        const text = graphemes.join("");
+        return requiresShaping(text) ? shape(text) : (additive(graphemes).at(-1) ?? 0);
+      }),
+      measureAdvances: createChunkedAdvanceMeasurer({
+        requiresShaping,
+        measureAdditive: additive,
+        measureShaped(graphemes) {
+          shape(graphemes.join(""));
+          let advance = 0;
+          return graphemes.map((grapheme) => (advance += monospaceWidth(grapheme)));
+        },
+      }),
+    },
+  };
+}
+
+function addedFile(file: BenchmarkCorpusFile): ParsedDiffFile {
+  return {
+    path: file.path,
+    isNew: true,
+    isDeleted: false,
+    additions: file.lines.length,
+    deletions: 0,
+    hunks: [
+      {
+        oldStart: 0,
+        oldCount: 0,
+        newStart: 1,
+        newCount: file.lines.length,
+        lines: file.lines.map((content) => ({
+          type: "add" as const,
+          content,
+          tokens: [{ text: content, style: null }],
+        })),
+      },
+    ],
+  };
+}
+
+interface Scenario {
+  name: string;
+  files: ParsedDiffFile[];
+}
+
+const SCENARIOS: Scenario[] = [
+  { name: "real code (diff-pane.tsx)", files: [addedFile(realCodeCorpusFile())] },
+  { name: "changelog (42 long prose lines)", files: [addedFile(changelogCorpusFile())] },
+  { name: "generated code (1200 lines)", files: [addedFile(generatedCodeCorpusFile(1200))] },
+  { name: "json (300 lines x 300 chars)", files: [addedFile(jsonCorpusFile(300))] },
+  { name: "pull request (all of the above)", files: pullRequestCorpus().map(addedFile) },
+];
+
+function input(
+  scenario: Scenario,
+  options: {
+    wrapLines: boolean;
+    viewport: { width: number; height: number };
+    measureText: TextMeasurer;
+  },
+): BuildDiffDocumentModelInput {
+  return {
+    files: scenario.files,
+    collapsedFilePaths: new Set(),
+    layout: "unified",
+    wrapLines: options.wrapLines,
+    viewportWidth: options.viewport.width,
+    typography: { family: "monospace", size: 13, lineHeight: 20 },
+    measureText: options.measureText,
+    palette: {
+      surface: "#000",
+      headerSurface: "#111",
+      border: "#222",
+      foreground: "#fff",
+      foregroundMuted: "#aaa",
+      addition: "green",
+      deletion: "red",
+      additionBackground: "#010",
+      deletionBackground: "#100",
+      emptyBackground: "#111",
+      selection: "blue",
+      headerActiveSurface: "#222",
+      headerBorder: "#333",
+      statusSuccess: "green",
+      statusDanger: "red",
+      statusWarning: "orange",
+      syntax: {},
+    },
+    labels: { binary: "Binary", tooLarge: "Too large" },
+    // What the surfaces pass at scrollTop 0: five viewports tall.
+    materializationWindow: { top: 0, height: options.viewport.height * 5 },
+  };
+}
+
+interface Variant {
+  name: string;
+  wrapLines: boolean;
+  viewport: { width: number; height: number };
+  backend: () => Backend;
+}
+
+const VARIANTS: Variant[] = [
+  {
+    name: "unwrapped desktop | web",
+    wrapLines: false,
+    viewport: DESKTOP_VIEWPORT,
+    backend: webBackend,
+  },
+  {
+    name: "wrapped desktop | web",
+    wrapLines: true,
+    viewport: DESKTOP_VIEWPORT,
+    backend: webBackend,
+  },
+  {
+    name: "wrapped desktop | web without width probe",
+    wrapLines: true,
+    viewport: DESKTOP_VIEWPORT,
+    backend: webBackendWithoutWidthProbe,
+  },
+  {
+    name: "wrapped phone | web",
+    wrapLines: true,
+    viewport: PHONE_VIEWPORT,
+    backend: webBackend,
+  },
+  {
+    name: "unwrapped desktop | native",
+    wrapLines: false,
+    viewport: DESKTOP_VIEWPORT,
+    backend: nativeBackend,
+  },
+  {
+    name: "wrapped desktop | native",
+    wrapLines: true,
+    viewport: DESKTOP_VIEWPORT,
+    backend: nativeBackend,
+  },
+  {
+    name: "wrapped phone | native",
+    wrapLines: true,
+    viewport: PHONE_VIEWPORT,
+    backend: nativeBackend,
+  },
+];
+
+function countModel(scenario: Scenario, variant: Variant) {
+  const { measurer, stats } = variant.backend();
+  const started = performance.now();
+  const model = buildDiffDocumentModel(
+    input(scenario, {
+      wrapLines: variant.wrapLines,
+      viewport: variant.viewport,
+      measureText: measurer,
+    }),
+  );
+  const elapsedMs = performance.now() - started;
+  let materializedCells = 0;
+  let fragments = 0;
+  let graphemes = 0;
+  for (const row of model.rows) {
+    if (row.kind !== "line") continue;
+    for (const cell of row.cells) {
+      if (!cell || cell.fragments.length === 0) continue;
+      materializedCells += 1;
+      fragments += cell.fragments.length;
+      for (const fragment of cell.fragments) graphemes += fragment.graphemes.length;
+    }
+  }
+  const sourceLines = scenario.files.reduce(
+    (count, file) => count + file.hunks.reduce((total, hunk) => total + hunk.lines.length, 0),
+    0,
+  );
+  return {
+    sourceLines,
+    materializedCells,
+    fragments,
+    graphemeObjects: graphemes,
+    measureCalls: stats.calls,
+    measuredCharacters: stats.characters,
+    coldMs: Number(elapsedMs.toFixed(1)),
+  };
+}
+
+const report: Record<string, ReturnType<typeof countModel>> = {};
+for (const scenario of SCENARIOS) {
+  for (const variant of VARIANTS) {
+    report[`${scenario.name} | ${variant.name}`] = countModel(scenario, variant);
+  }
+}
+console.table(report);
+
+for (const scenario of SCENARIOS) {
+  describe(scenario.name, () => {
+    for (const variant of VARIANTS) {
+      if (variant.backend === nativeBackend) continue;
+      bench(variant.name, () => {
+        buildDiffDocumentModel(
+          input(scenario, {
+            wrapLines: variant.wrapLines,
+            viewport: variant.viewport,
+            measureText: variant.backend().measurer,
+          }),
+        );
+      });
+    }
+  });
+}
@@ -10,7 +10,7 @@ import {
  resolveRelayoutScrollTop,
  shouldApplyRelayoutScroll,
 } from "./model";
-import type { BuildDiffDocumentModelInput, TextMeasurer } from "./types";
+import type { BuildDiffDocumentModelInput, DiffCell, TextMeasurer } from "./types";

 const measurer: TextMeasurer = { measure: (text) => Array.from(text).length * 10 };

@@ -639,17 +639,119 @@ describe("diff document model", () => {
    expect(firstLineCells(last, "b.ts")).toBe(firstLineCells(first, "b.ts"));
  });

-  it("keeps wrapped geometry fully measured", () => {
+  it("replaces estimated wrapped heights through reuse and preserves the scroll anchor", () => {
+    const changed = fileWithChangedLine("x".repeat(100));
+    const files = [changed, file("later.ts")];
+    // Deliberately inaccurate M width makes the estimate differ from exact layout.
+    const measureText = {
+      measure: (text: string) => (text === "M" ? 5 : Array.from(text).length * 10),
+    };
+    const options = input({ files, measureText });
+    const first = buildDiffDocumentModel({
+      ...options,
+      materializationWindow: { top: 30, height: 1 },
+    });
+    const eager = buildDiffDocumentModel(options);
+    const exact = buildDiffDocumentModel({ ...options, reuseFrom: [first] });
+    expect(first.height).not.toBe(eager.height);
+    expect(exact.rows).toEqual(eager.rows);
+    expect(exact.height).toBe(eager.height);
+    expect(firstLineCells(exact, changed.path)).toBe(firstLineCells(first, changed.path));
+    const scrollTop = first.files[1]!.bodyTop + 3;
+    const nextScrollTop = resolveRelayoutScrollTop(first, exact, scrollTop);
+    expect(nextScrollTop).toBe(exact.files[1]!.bodyTop + 3);
+    expect(captureScrollAnchor(exact, nextScrollTop)).toEqual(
+      captureScrollAnchor(first, scrollTop),
+    );
+  });
+
+  it("preserves the wrapped source offset when resize leaves the anchor outside the window", () => {
+    const source = file();
+    source.hunks[0]!.lines = Array.from({ length: 200 }, () => ({
+      type: "add" as const,
+      content: "x".repeat(200),
+    }));
+    const options = input({ files: [source], viewportWidth: 500 });
+    const before = buildDiffDocumentModel(options);
+    const scrollTop = before.rows[100]!.top + 36;
+    const anchor = captureScrollAnchor(before, scrollTop)!;
+    expect(anchor).toMatchObject({ kind: "text", cellOffset: 90 });
+    const resizedOptions = { ...options, viewportWidth: 180 };
+    const estimated = buildDiffDocumentModel({
+      ...resizedOptions,
+      materializationWindow: {
+        top: Math.floor(scrollTop / 800) * 800 - 800,
+        height: 2000,
+      },
+    });
+    const estimatedRow = estimated.rows[100]!;
+    expect(estimatedRow.kind === "line" && estimatedRow.cells[0]?.fragments).toEqual([]);
+    const restored = resolveRelayoutScrollTop(before, estimated, scrollTop);
+    const recaptured = captureScrollAnchor(estimated, restored)!;
+    expect(recaptured).toMatchObject({ kind: "text", cellOffset: 78 });
+
+    const materialized = buildDiffDocumentModel({
+      ...resizedOptions,
+      materializationWindow: { top: restored - 800, height: 2000 },
+      reuseFrom: [estimated],
+    });
+    const eager = buildDiffDocumentModel(resizedOptions);
+    const expected = resolveRelayoutScrollTop(before, eager, scrollTop);
+    expect(expected).toBe(28_938);
+    expect(Math.abs(restored - expected)).toBeLessThan(before.lineHeight);
+    expect(resolveRelayoutScrollTop(estimated, materialized, restored)).toBe(expected);
+  });
+
+  it("materializes the relayout anchor even when off-screen height estimates are inaccurate", () => {
+    const source = file();
+    source.hunks[0]!.lines = Array.from({ length: 200 }, () => ({
+      type: "add" as const,
+      content: "x".repeat(200),
+    }));
+    const options = input({ files: [source], viewportWidth: 500 });
+    const before = buildDiffDocumentModel(options);
+    const scrollTop = before.rows[100]!.top + 38;
+    const anchor = captureScrollAnchor(before, scrollTop)!;
+    const resizedOptions = {
+      ...options,
+      viewportWidth: 180,
+      measureText: { measure: (text: string) => (text === "M" ? 5 : text.length * 10) },
+    };
+    const resized = buildDiffDocumentModel({
+      ...resizedOptions,
+      materializationWindow: { top: scrollTop - 800, height: 2000 },
+      materializationAnchor: anchor,
+    });
+    const row = resized.rows[100]!;
+    expect(row.kind === "line" && row.cells[0]!.fragments.length).toBeGreaterThan(0);
+    const restored = resolveRelayoutScrollTop(before, resized, scrollTop);
+    const materialized = buildDiffDocumentModel({
+      ...resizedOptions,
+      materializationWindow: { top: restored - 800, height: 2000 },
+      materializationAnchor: captureScrollAnchor(resized, restored),
+      reuseFrom: [resized],
+    });
+    const nextScrollTop = resolveRelayoutScrollTop(resized, materialized, restored);
+    const nextAnchor = captureScrollAnchor(materialized, nextScrollTop);
+    // Offset 90 belongs to the fragment beginning at 78 after narrowing.
+    expect(nextAnchor).toMatchObject({ kind: "text", cellOffset: 78, viewportDelta: -2 });
+    expect(nextScrollTop).toBe(resolveScrollAnchor(materialized, anchor));
+  });
+
+  it("materializes wrapped text only inside the window", () => {
    const model = buildDiffDocumentModel(
      input({
        files: [file("a.ts"), file("b.ts")],
        wrapLines: true,
-        materializationWindow: { top: 0, height: FILE_HEADER_HEIGHT },
+        materializationWindow: { top: FILE_HEADER_HEIGHT, height: 1 },
      }),
    );

    expect(measuredFragments(model, "a.ts")).toBeGreaterThan(0);
-    expect(measuredFragments(model, "b.ts")).toBeGreaterThan(0);
+    expect(measuredFragments(model, "b.ts")).toBe(0);
+    expect(
+      model.rows.filter((row) => row.kind === "line" && row.cells.some(hasFragments)),
+    ).toHaveLength(1);
  });
 });

@@ -798,3 +900,7 @@ function reviewActionsWithEditor(
    onDeleteComment() {},
  };
 }
+
+function hasFragments(cell: DiffCell | null): boolean {
+  return cell !== null && cell.fragments.length > 0;
+}
@@ -154,27 +154,38 @@ function appendReusableFileRows(candidate: {
 }): FileRowsResult {
  const reusedRows = candidate.model.rows.slice(candidate.file.rowStart, candidate.file.rowEnd);
  const retainsRowGeometry =
-    !candidate.input.wrapLines &&
    candidate.model.reviewGeometryKey === candidate.reviewGeometryKey &&
    candidate.file.top === candidate.fileTop &&
    candidate.file.fileIndex === candidate.fileIndex &&
    candidate.file.rowStart === candidate.rows.length;
  let top = candidate.bodyTop;
  for (const reusedRow of reusedRows) {
-    let nextRow = retainsRowGeometry ? reusedRow : repositionReusedRow(reusedRow, candidate, top);
+    let nextRow =
+      retainsRowGeometry && reusedRow.top === top
+        ? reusedRow
+        : repositionReusedRow(reusedRow, candidate, top);
    if (
      nextRow.kind === "line" &&
-      intersectsMaterializationWindow(candidate.input, top, top + nextRow.height) &&
+      (intersectsMaterializationWindow(candidate.input, top, top + nextRow.height) ||
+        containsMaterializationAnchor(candidate.input, candidate.file.path, nextRow.cells)) &&
      nextRow.cells.some((cell) => cell && cell.fragments.length === 0)
    ) {
+      const cells = materializeCells(
+        nextRow.cells,
+        candidate.file.file,
+        candidate.file.gutterWidth,
+        candidate.input,
+      );
      nextRow = {
        ...nextRow,
-        cells: materializeCells(
-          nextRow.cells,
-          candidate.file.file,
-          candidate.file.gutterWidth,
-          candidate.input,
-        ),
+        cells,
+        height:
+          Math.max(
+            candidate.input.typography.lineHeight,
+            ...cells.map(
+              (cell) => (cell?.fragments.length ?? 1) * candidate.input.typography.lineHeight,
+            ),
+          ) + nextRow.reviewHeight,
      };
    }
    candidate.rows.push(nextRow);
@@ -240,6 +251,7 @@ function appendNewFileRows(candidate: {
  const lines = geometryLines(
    lineSources(candidate.file, candidate.input.layout, false),
    candidate.input,
+    candidate.gutterWidth,
  );
  const fileBottom =
    candidate.bodyTop +
@@ -252,8 +264,8 @@ function appendNewFileRows(candidate: {
  let maximumHorizontalOverflow = 0;
  for (const line of lines) {
    const shouldMaterialize =
-      candidate.input.wrapLines ||
-      intersectsMaterializationWindow(candidate.input, top, top + line.height);
+      intersectsMaterializationWindow(candidate.input, top, top + line.height) ||
+      containsMaterializationAnchor(candidate.input, candidate.file.path, line.cells);
    const cells = shouldMaterialize
      ? materializeCells(line.cells, candidate.file, candidate.gutterWidth, candidate.input)
      : line.cells;
@@ -275,7 +287,7 @@ function appendNewFileRows(candidate: {
      candidate.input.typography.lineHeight,
      ...cells.map((cell) => (cell?.fragments.length ?? 1) * candidate.input.typography.lineHeight),
    );
-    const height = textHeight + line.reviewHeight;
+    const height = shouldMaterialize ? textHeight + line.reviewHeight : line.height;
    candidate.rows.push({
      kind: "line",
      index: candidate.rows.length,
@@ -412,13 +424,14 @@ function cellSource(line: SplitDiffDisplayLine | null, includeTokens: boolean):
  };
 }

-function geometryCell(source: CellSource): DiffCell {
+function geometryCell(source: CellSource, estimatedWrapColumns: number): DiffCell {
  return {
    type: source.type,
    content: source.content,
    lineNumber: source.lineNumber,
    tokens: [],
    fragments: [],
+    estimatedWrapColumns,
    reviewTarget: source.reviewTarget,
    sourceIdentity: source.sourceIdentity,
  };
@@ -427,20 +440,87 @@ function geometryCell(source: CellSource): DiffCell {
 function geometryLines(
  sources: Array<[CellSource] | [CellSource | null, CellSource | null]>,
  input: BuildDiffDocumentModelInput,
+  gutterWidth: number,
 ): GeometryLine[] {
+  const characterWidth = input.wrapLines ? input.measureText.measure("M") : 0;
  return sources.map((sourceCells) => {
+    const columns = input.wrapLines
+      ? Math.max(
+          1,
+          Math.floor(
+            (input.viewportWidth / sourceCells.length - gutterWidth - CODE_HORIZONTAL_PADDING) /
+              characterWidth,
+          ),
+        )
+      : Infinity;
    const cells = sourceCells.map((source) =>
-      source ? geometryCell(source) : null,
+      source ? geometryCell(source, columns) : null,
    ) as DiffLineRow["cells"];
    const reviewHeight = reviewHeightForCells(cells, input);
    return {
      cells,
      reviewHeight,
-      height: input.typography.lineHeight + reviewHeight,
+      // Unseen wrapped rows use a monospace estimate; materialization replaces
+      // it with the exact fragment count without changing source identity.
+      height:
+        input.typography.lineHeight *
+          Math.max(
+            1,
+            ...cells.map((cell) => {
+              return cell && input.wrapLines
+                ? Math.ceil(estimatedDisplayOffset(cell.content) / columns)
+                : 1;
+            }),
+          ) +
+        reviewHeight,
    };
  });
 }

+// Use the same tab expansion and column count for estimated heights and anchors.
+function estimatedDisplayOffset(text: string): number {
+  return text.length + (text.match(/\t/g)?.length ?? 0) * 3;
+}
+
+function estimatedVisualLine(cell: DiffCell, sourceOffset: number): number {
+  return Math.floor(
+    estimatedDisplayOffset(cell.content.slice(0, sourceOffset)) /
+      (cell.estimatedWrapColumns ?? Infinity),
+  );
+}
+
+function estimatedFragment(cell: DiffCell, visualIndex: number, lineHeight: number) {
+  const columns = cell.estimatedWrapColumns ?? Infinity;
+  const lastIndex = Math.max(0, Math.ceil(estimatedDisplayOffset(cell.content) / columns) - 1);
+  const index = Math.min(visualIndex, lastIndex);
+  const displayOffset = index === 0 ? 0 : index * columns;
+  let start = Math.min(displayOffset, cell.content.length);
+  if (cell.content.includes("\t")) {
+    let offset = 0;
+    start = 0;
+    while (start < cell.content.length && offset < displayOffset) {
+      offset += cell.content[start] === "\t" ? 4 : 1;
+      start += 1;
+    }
+  }
+  return { start, top: index * lineHeight };
+}
+
+// A resize can move the anchor outside the old window. Measure that row in the
+// same pass: estimated columns cannot identify exact shaped-text fragments.
+function containsMaterializationAnchor(
+  input: BuildDiffDocumentModelInput,
+  path: string,
+  cells: DiffLineRow["cells"],
+): boolean {
+  const anchor = input.materializationAnchor;
+  return (
+    anchor?.kind === "text" &&
+    anchor.path === path &&
+    cells.some((cell) => cell && sameSourceIdentity(cell.sourceIdentity, anchor.sourceIdentity))
+  );
+}
+
 function intersectsMaterializationWindow(
  input: BuildDiffDocumentModelInput,
  fileTop: number,
@@ -795,17 +875,27 @@ export function captureScrollAnchor(
  }
  const textY = Math.max(0, Math.min(scrollTop - row.top, row.height - row.reviewHeight));
  const visualIndex = Math.max(0, Math.floor(textY / model.lineHeight));
-  let cellIndex = row.cells.findIndex((cell) => Boolean(cell?.fragments[visualIndex]));
+  let cellIndex = row.cells.findIndex(
+    (cell) =>
+      cell &&
+      (cell.fragments.length > 0
+        ? Boolean(cell.fragments[visualIndex])
+        : estimatedFragment(cell, visualIndex, model.lineHeight).top ===
+          visualIndex * model.lineHeight),
+  );
  if (cellIndex < 0) cellIndex = row.cells.findIndex((cell) => cell !== null);
  const cell = row.cells[cellIndex];
  if (!cell) return { kind: "header", path: file.path, viewportDelta: row.top - scrollTop };
-  const fragment = cell.fragments[Math.min(cell.fragments.length - 1, visualIndex)];
-  const anchorY = row.top + (fragment?.top ?? 0);
+  const fragment =
+    cell.fragments.length > 0
+      ? cell.fragments[Math.min(cell.fragments.length - 1, visualIndex)]!
+      : estimatedFragment(cell, visualIndex, model.lineHeight);
+  const anchorY = row.top + fragment.top;
  return {
    kind: "text",
    path: file.path,
    sourceIdentity: cell.sourceIdentity,
-    cellOffset: fragment?.start ?? 0,
+    cellOffset: fragment.start,
    viewportDelta: anchorY - scrollTop,
  };
 }
@@ -831,8 +921,10 @@ export function resolveScrollAnchor(model: DiffDocumentModel, anchor: DiffScroll
  const fragment =
    cell.fragments.find(
      (candidate) => candidate.start <= anchor.cellOffset && anchor.cellOffset < candidate.end,
-    ) ?? cell.fragments.at(-1);
-  return Math.max(0, row.top + (fragment?.top ?? 0) - anchor.viewportDelta);
+    ) ??
+    cell.fragments.at(-1) ??
+    estimatedFragment(cell, estimatedVisualLine(cell, anchor.cellOffset), model.lineHeight);
+  return Math.max(0, row.top + fragment.top - anchor.viewportDelta);
 }

 export function resolveRelayoutScrollTop(
@@ -38,6 +38,7 @@ import {
 } from "./horizontal-offsets";
 import {
  buildDiffDocumentModel,
+  captureScrollAnchor,
  resolveRelayoutScrollTop,
  retainReusableModels,
  shouldApplyRelayoutScroll,
@@ -138,6 +139,9 @@ export function DiffSurface(props: DiffSurfaceProps) {
        tooLarge: t("workspace.git.diff.tooLarge"),
      },
      materializationWindow: diffMaterializationWindow(fileWindowTop, viewport.height),
+      materializationAnchor: previousModelRef.current
+        ? captureScrollAnchor(previousModelRef.current, scrollTop.get())
+        : null,
      reuseFrom,
    });
    reusableModelRef.current = {
@@ -148,6 +152,7 @@ export function DiffSurface(props: DiffSurfaceProps) {
  }, [
    measurement,
    fileWindowTop,
+    scrollTop,
    props.collapsedFilePaths,
    props.displayPreferences.layout,
    props.displayPreferences.wrapLines,
@@ -28,10 +28,20 @@ import {
 } from "./hit-testing";
 import { retainHorizontalOffsetMapForPaths } from "./horizontal-offsets";
 import { HorizontalScroll } from "./horizontal-scroll.web";
-import { buildDiffDocumentModel, FILE_HEADER_HEIGHT, resolveRelayoutScrollTop } from "./model";
+import {
+  buildDiffDocumentModel,
+  captureScrollAnchor,
+  FILE_HEADER_HEIGHT,
+  resolveRelayoutScrollTop,
+} from "./model";
 import { paintWebFileHeader, paintWebViewport } from "./paint.web";
 import { hasPointerDragStarted } from "./pointer-gesture";
-import { createMeasuredAdvances } from "./text-measurement";
+import {
+  createMeasuredAdvances,
+  createAdditiveAdvances,
+  createChunkedWidthMeasurer,
+  requiresShaping,
+} from "./text-measurement";
 import { retainDiffViewport } from "./viewport";
 import type {
  DiffHit,
@@ -183,6 +193,9 @@ export function DiffSurface(props: DiffSurfaceProps) {
        tooLarge: t("workspace.git.diff.tooLarge"),
      },
      materializationWindow: diffMaterializationWindow(fileWindowTop, viewport.height),
+      materializationAnchor: previousModelRef.current
+        ? captureScrollAnchor(previousModelRef.current, scrollTopRef.current)
+        : null,
    });
    return next;
  }, [
@@ -1166,5 +1179,13 @@ function createWebTextMeasurer(typography: DiffTypography): TextMeasurer {
  };
  // Canvas exposes no glyph coverage, so `requiresShaping` alone decides which
  // runs a shaper has to see -- there is no `glyphIds` to fall back on.
-  return { measure, measureAdvances: createMeasuredAdvances((text) => measure(text)) };
+  const additive = createAdditiveAdvances(measure);
+  return {
+    measure,
+    measureAdvances: createMeasuredAdvances(measure),
+    measureWidth: createChunkedWidthMeasurer((graphemes) => {
+      const text = graphemes.join("");
+      return requiresShaping(text) ? measure(text) : (additive(graphemes).at(-1) ?? 0);
+    }),
+  };
 }
@@ -112,6 +112,8 @@ export interface DiffCell {
  lineNumber: number | null;
  tokens: DiffTokenRun[];
  fragments: DiffFragment[];
+  /** Monospace columns per visual line until this cell is measured. */
+  estimatedWrapColumns?: number;
  reviewTarget: ReviewableDiffTarget | null;
  sourceIdentity: DiffSourceIdentity;
 }
@@ -185,6 +187,8 @@ export interface BuildDiffDocumentModelInput {
  reviewActions?: InlineReviewActions;
  labels: { binary: string; tooLarge: string };
  materializationWindow?: { top: number; height: number };
+  /** Also measure the anchored row so relayout can restore its exact fragment. */
+  materializationAnchor?: DiffScrollAnchor | null;
  /** A geometry-compatible model whose unchanged file measurements may be reused. */
  reuseFrom?: readonly DiffDocumentModel[];
 }
@@ -95,6 +95,37 @@ describe("diff document workspace cache", () => {
    expect(stats.calls).toBe(0);
  });

+  it("materializes a new anchor when the viewport window is unchanged", () => {
+    const cache = createDiffDocumentWorkspaceCache();
+    const source = diffFile();
+    source.hunks[0]!.lines = Array.from({ length: 100 }, () => ({
+      type: "add" as const,
+      content: "x".repeat(200),
+    }));
+    const { measureText } = countingMeasurer();
+    const input = modelInput([source], measureText, {
+      wrapLines: true,
+      materializationWindow: { top: 30, height: 54 },
+    });
+    const first = cache.buildModel(input);
+    const anchored = cache.buildModel({
+      ...input,
+      materializationAnchor: {
+        kind: "text",
+        path: source.path,
+        sourceIdentity: { hunkIndex: 0, lineIndex: 50, side: "new" },
+        cellOffset: 90,
+        viewportDelta: -2,
+      },
+    });
+    const firstRow = first.rows[50]!;
+    const anchoredRow = anchored.rows[50]!;
+    expect(firstRow.kind === "line" && firstRow.cells[0]!.fragments.length).toBe(0);
+    expect(anchoredRow.kind === "line" && anchoredRow.cells[0]!.fragments.length).toBeGreaterThan(
+      0,
+    );
+  });
+
  it("retains text cells for unchanged files when another file changes", () => {
    const cache = createDiffDocumentWorkspaceCache();
    const { measureText } = countingMeasurer();
@@ -128,7 +128,9 @@ function exactModelKey(input: Omit<BuildDiffDocumentModelInput, "reuseFrom">): s
  const materializationWindow = input.materializationWindow
    ? [input.materializationWindow.top, input.materializationWindow.height]
    : null;
-  return JSON.stringify([collapsedFilePaths, reviewGeometry, materializationWindow]);
+  const anchor = input.materializationAnchor;
+  const anchoredRow = anchor?.kind === "text" ? [anchor.path, anchor.sourceIdentity] : null;
+  return JSON.stringify([collapsedFilePaths, reviewGeometry, materializationWindow, anchoredRow]);
 }

 function typographyKey(typography: DiffTypography): string {