fix(tui): cursor position tracking when content shrinks with unchanged lines

packages/tui/test/tui-render.test.ts

@@ -0,0 +1,43 @@
+import assert from "node:assert";
+import { describe, it } from "node:test";
+import { type Component, TUI } from "../src/tui.js";
+import { VirtualTerminal } from "./virtual-terminal.js";
+
+class TestComponent implements Component {
+	lines: string[] = [];
+	render(_width: number): string[] {
+		return this.lines;
+	}
+	invalidate(): void {}
+}
+
+describe("TUI differential rendering", () => {
+	it("tracks cursor correctly when content shrinks with unchanged remaining lines", async () => {
+		const terminal = new VirtualTerminal(40, 10);
+		const tui = new TUI(terminal);
+		const component = new TestComponent();
+		tui.addChild(component);
+
+		// Initial render: 5 identical lines
+		component.lines = ["Line 0", "Line 1", "Line 2", "Line 3", "Line 4"];
+		tui.start();
+		await terminal.flush();
+
+		// Shrink to 3 lines, all identical to before (no content changes in remaining lines)
+		component.lines = ["Line 0", "Line 1", "Line 2"];
+		tui.requestRender();
+		await terminal.flush();
+
+		// cursorRow should be 2 (last line of new content)
+		// Verify by doing another render with a change on line 1
+		component.lines = ["Line 0", "CHANGED", "Line 2"];
+		tui.requestRender();
+		await terminal.flush();
+
+		const viewport = terminal.getViewport();
+		// Line 1 should show "CHANGED", proving cursor tracking was correct
+		assert.ok(viewport[1]?.includes("CHANGED"), `Expected "CHANGED" on line 1, got: ${viewport[1]}`);
+
+		tui.stop();
+	});
+});