make unit tests faster (#241847)

test/unit/browser/renderer.html

@@ -34,6 +34,46 @@
 		const urlParams = new URLSearchParams(window.location.search);
 		const isCI = urlParams.get('ci');
 
+		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);
+		})();
+
+		Mocha.Runner.immediately = setTimeout0;
+
 		mocha.setup({
 			ui: 'tdd',
 			timeout: isCI ? 30000 : 5000