Make large diffs open quickly and scroll as one surface (#3212)

* perf(app): virtualize large diff rendering

Render bounded diff row groups through one vertical list and one horizontal surface. Bound retained list ranges when expansion grows the data set so large diffs paint promptly without sacrificing scrolling behavior.

* refactor(app): normalize working tree diff options

* fix(app): keep wide diff text reachable

packages/app/src/utils/diff-rendering.test.ts

@@ -1,6 +1,7 @@
 import { describe, expect, it } from "vitest";
 
 import {
+  compactHighlightTokens,
   formatDiffContentText,
   formatDiffGutterText,
   hasVisibleDiffTokens,
@@ -28,4 +29,23 @@ describe("diff-rendering", () => {
     expect(hasVisibleDiffTokens([{ text: "" }])).toBe(false);
     expect(hasVisibleDiffTokens([{ text: "const value = 1;" }])).toBe(true);
   });
+
+  it("compacts adjacent syntax tokens without changing their text or styles", () => {
+    expect(
+      compactHighlightTokens([
+        { text: "const", style: "keyword" },
+        { text: " ", style: null },
+        { text: "value", style: null },
+        { text: " ", style: null },
+        { text: "=", style: "operator" },
+        { text: "", style: "operator" },
+        { text: " 1", style: "number" },
+      ]),
+    ).toEqual([
+      { text: "const", style: "keyword" },
+      { text: " value ", style: null },
+      { text: "=", style: "operator" },
+      { text: " 1", style: "number" },
+    ]);
+  });
 });