make unit tests faster (#241847)

test/unit/electron/renderer.js

@@ -398,7 +398,57 @@ class IPCReporter {
 	}
 }
 
+const $globalThis = globalThis;
+const setTimeout0IsFaster = (typeof $globalThis.postMessage === 'function' && !$globalThis.importScripts);
+
+/**
+ * See https://html.spec.whatwg.org/multipage/timers-and-user-prompts.html#:~:text=than%204%2C%20then-,set%20timeout%20to%204,-.
+ *
+ * Works similarly to `setTimeout(0)` but doesn't suffer from the 4ms artificial delay
+ * that browsers set when the nesting level is > 5.
+ */
+const setTimeout0 = (() => {
+	if (setTimeout0IsFaster) {
+		const pending = [];
+
+		$globalThis.addEventListener('message', (e) => {
+			if (e.data && e.data.vscodeScheduleAsyncWork) {
+				for (let i = 0, len = pending.length; i < len; i++) {
+					const candidate = pending[i];
+					if (candidate.id === e.data.vscodeScheduleAsyncWork) {
+						pending.splice(i, 1);
+						candidate.callback();
+						return;
+					}
+				}
+			}
+		});
+		let lastId = 0;
+		return (callback) => {
+			const myId = ++lastId;
+			pending.push({
+				id: myId,
+				callback: callback
+			});
+			$globalThis.postMessage({ vscodeScheduleAsyncWork: myId }, '*');
+		};
+	}
+	return (callback) => setTimeout(callback);
+})();
+
 async function runTests(opts) {
+	// @ts-expect-error
+	Mocha.Runner.immediately = setTimeout0;
+
+	mocha.setup({
+		ui: 'tdd',
+		// @ts-expect-error
+		reporter: opts.dev ? 'html' : IPCReporter,
+		grep: opts.grep,
+		timeout: opts.timeout ?? (IS_CI ? 30000 : 5000),
+		forbidOnly: IS_CI // disallow .only() when running on build machine
+	});
+
 	// this *must* come before loadTests, or it doesn't work.
 	if (opts.timeout !== undefined) {
 		mocha.timeout(opts.timeout);
@@ -406,15 +456,6 @@ async function runTests(opts) {
 
 	await loadTests(opts);
 
-	if (opts.grep) {
-		mocha.grep(opts.grep);
-	}
-
-	if (!opts.dev) {
-		// @ts-expect-error
-		mocha.reporter(IPCReporter);
-	}
-
 	const runner = mocha.run(async () => {
 		await createCoverageReport(opts)
 		ipcRenderer.send('all done');