perf(tui): reuse prepared transcript render lines (#127767)

Amp-Thread-ID: https://ampcode.com/threads/T-01a02570-1023-77c8-9513-e43f179ee673

Co-authored-by: Amp <amp@ampcode.com>

src/tui/components/hyperlink-markdown.test.ts

@@ -17,6 +17,30 @@ function osc8Targets(raw: string) {
 }
 
 describe("HyperlinkMarkdown", () => {
+  it("does not reallocate prepared lines for an unchanged same-width redraw", () => {
+    const markdown = new HyperlinkMarkdown(
+      "مرحبا [docs](https://example.test/path)",
+      0,
+      0,
+      markdownTheme,
+    );
+
+    const first = markdown.render(80);
+    expect(markdown.render(80)).toBe(first);
+
+    const resized = markdown.render(40);
+    expect(resized).not.toBe(first);
+    expect(markdown.render(40)).toBe(resized);
+
+    markdown.setText("updated");
+    const updated = markdown.render(40);
+    expect(updated).not.toBe(resized);
+    expect(markdown.render(40)).toBe(updated);
+
+    markdown.invalidate();
+    expect(markdown.render(40)).not.toBe(updated);
+  });
+
   it("moves dunder identifiers intact across fenced code wrap boundaries", () => {
     const markdown = new HyperlinkMarkdown(
       ["```python", 'if __name__ == "__main__":', "```"].join("\n"),

src/tui/components/hyperlink-markdown.ts

@@ -24,6 +24,7 @@ function sanitizeMarkdownDisplayText(text: string): string {
 export class HyperlinkMarkdown implements Component {
   private inner: Markdown;
   private urls: string[];
+  private cachedRender?: { width: number; lines: string[] };
 
   constructor(
     text: string,
@@ -39,16 +40,25 @@ export class HyperlinkMarkdown implements Component {
   }
 
   render(width: number): string[] {
-    return addOsc8Hyperlinks(this.inner.render(width), this.urls).map(isolateRtlRenderedLine);
+    if (this.cachedRender?.width === width) {
+      return this.cachedRender.lines;
+    }
+    const lines = addOsc8Hyperlinks(this.inner.render(width), this.urls).map(
+      isolateRtlRenderedLine,
+    );
+    this.cachedRender = { width, lines };
+    return lines;
   }
 
   setText(text: string): void {
     const displayText = sanitizeMarkdownDisplayText(text);
     this.inner.setText(displayText);
     this.urls = extractUrls(displayText);
+    this.cachedRender = undefined;
   }
 
   invalidate(): void {
     this.inner.invalidate();
+    this.cachedRender = undefined;
   }
 }