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.ts

@@ -2,6 +2,28 @@ interface HighlightLikeToken {
   text: string;
 }
 
+interface StyledHighlightToken extends HighlightLikeToken {
+  style: string | null;
+}
+
+export function compactHighlightTokens(
+  tokens: readonly StyledHighlightToken[],
+): StyledHighlightToken[] {
+  const compacted: StyledHighlightToken[] = [];
+  for (const token of tokens) {
+    if (token.text.length === 0) {
+      continue;
+    }
+    const previous = compacted.at(-1);
+    if (previous?.style === token.style) {
+      previous.text += token.text;
+      continue;
+    }
+    compacted.push({ text: token.text, style: token.style });
+  }
+  return compacted;
+}
+
 // Preserve row height when a gutter or diff cell is intentionally blank.
 // Non-breaking space because the gutter <Text> uses numberOfLines={1}, which
 // collapses a plain ASCII space to zero height on web.