Reduce stalls when opening large diffs (#4574)

* perf(app): reduce avoidable work when opening large diffs

Large snapshots were recursively reconstructed for reference stability, while native scroll worklets captured complete file models. Preserve identity at the data producers and prepare text for the visible window without rebuilding unchanged files.

* test(app): await commit diff query completion without polling

* chore(app): remove profiling artifacts from the source tree

packages/app/src/git/diff-document/model.test.ts

@@ -71,6 +71,55 @@ function input(overrides: Partial<BuildDiffDocumentModelInput> = {}): BuildDiffD
 }
 
 describe("diff document model", () => {
+  it("materializes only visible rows of a large file while preserving full horizontal extent", () => {
+    const source = file("large.ts");
+    source.hunks[0]!.lines = Array.from({ length: 1000 }, (_, index) => ({
+      type: "add",
+      content: index === 900 ? "a very wide offscreen line ".repeat(20) : `line ${index}`,
+      tokens: [{ text: `line ${index}`, style: null }],
+    }));
+    let advancedCharacters = 0;
+    const measureText: TextMeasurer = {
+      measure: (text) => text.length * 10,
+      measureAdvances(graphemes) {
+        advancedCharacters += graphemes.join("").length;
+        return graphemes.map((_, index) => (index + 1) * 10);
+      },
+    };
+    const base = input({ files: [source], wrapLines: false, viewportWidth: 300, measureText });
+    const eager = buildDiffDocumentModel(base);
+    advancedCharacters = 0;
+    const first = buildDiffDocumentModel({
+      ...base,
+      materializationWindow: { top: FILE_HEADER_HEIGHT, height: 54 },
+    });
+    expect(measuredFragments(first, source.path)).toBe(3);
+    expect(advancedCharacters).toBeLessThan(100);
+    expect(first.height).toBe(eager.height);
+    expect(first.files[0]!.contentWidth).toBe(eager.files[0]!.contentWidth);
+    const priorCells = firstLineCells(first, source.path);
+    advancedCharacters = 0;
+    const next = buildDiffDocumentModel({
+      ...base,
+      reuseFrom: [first],
+      materializationWindow: { top: FILE_HEADER_HEIGHT + 900 * 18, height: 54 },
+    });
+    expect(firstLineCells(next, source.path)).toBe(priorCells);
+    expect(next.rows[900]).toMatchObject({
+      kind: "line",
+      cells: [{ fragments: [{ text: source.hunks[0]!.lines[900]!.content }] }],
+    });
+    expect(advancedCharacters).toBeLessThan(600);
+    expect(next.height).toBe(eager.height);
+    advancedCharacters = 0;
+    buildDiffDocumentModel({
+      ...base,
+      reuseFrom: [next],
+      materializationWindow: { top: FILE_HEADER_HEIGHT, height: 54 },
+    });
+    expect(advancedCharacters).toBe(0);
+  });
+
   it("does not imperatively scroll for a no-op relayout", () => {
     expect(shouldApplyRelayoutScroll(320, 320)).toBe(false);
     expect(shouldApplyRelayoutScroll(320, 320.4)).toBe(false);