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

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();
	});
});