test(tui): cover style reset cases

packages/tui/test/tui-overlay-style-leak.test.ts

@@ -0,0 +1,72 @@
+import assert from "node:assert";
+import { describe, it } from "node:test";
+import type { Terminal as XtermTerminalType } from "@xterm/headless";
+import { type Component, TUI } from "../src/tui.js";
+import { VirtualTerminal } from "./virtual-terminal.js";
+
+class StaticLines implements Component {
+	constructor(private readonly lines: string[]) {}
+
+	render(): string[] {
+		return this.lines;
+	}
+
+	invalidate(): void {}
+}
+
+class StaticOverlay implements Component {
+	constructor(private readonly line: string) {}
+
+	render(): string[] {
+		return [this.line];
+	}
+
+	invalidate(): void {}
+}
+
+function getCellItalic(terminal: VirtualTerminal, row: number, col: number): number {
+	const xterm = (terminal as unknown as { xterm: XtermTerminalType }).xterm;
+	const buffer = xterm.buffer.active;
+	const line = buffer.getLine(buffer.viewportY + row);
+	assert.ok(line, `Missing buffer line at row ${row}`);
+	const cell = line.getCell(col);
+	assert.ok(cell, `Missing cell at row ${row} col ${col}`);
+	return cell.isItalic();
+}
+
+async function renderAndFlush(tui: TUI, terminal: VirtualTerminal): Promise<void> {
+	tui.requestRender(true);
+	await new Promise<void>((resolve) => process.nextTick(resolve));
+	await terminal.flush();
+}
+
+describe("TUI overlay compositing", () => {
+	it("should not leak styles when a trailing reset sits beyond the last visible column (no overlay)", async () => {
+		const width = 20;
+		const baseLine = `\x1b[3m${"X".repeat(width)}\x1b[23m`;
+
+		const terminal = new VirtualTerminal(width, 6);
+		const tui = new TUI(terminal);
+		tui.addChild(new StaticLines([baseLine, "INPUT"]));
+		tui.start();
+		await renderAndFlush(tui, terminal);
+		assert.strictEqual(getCellItalic(terminal, 1, 0), 0);
+		tui.stop();
+	});
+
+	it("should not leak styles when overlay slicing drops trailing SGR resets", async () => {
+		const width = 20;
+		const baseLine = `\x1b[3m${"X".repeat(width)}\x1b[23m`;
+
+		const terminal = new VirtualTerminal(width, 6);
+		const tui = new TUI(terminal);
+		tui.addChild(new StaticLines([baseLine, "INPUT"]));
+
+		tui.showOverlay(new StaticOverlay("OVR"), { row: 0, col: 5, width: 3 });
+		tui.start();
+		await renderAndFlush(tui, terminal);
+
+		assert.strictEqual(getCellItalic(terminal, 1, 0), 0);
+		tui.stop();
+	});
+});