event.once should beware of sync fires

src/vs/base/common/event.ts

@@ -305,19 +305,22 @@ export function toNativePromise<T>(event: Event<T>): Thenable<T> {
 
 export function once<T>(event: Event<T>): Event<T> {
 	return (listener, thisArgs = null, disposables?) => {
-		let sync = false;
+		// we need this, in case the event fires during the listener call
+		let didFire = false;
 
 		const result = event(e => {
-			if (result) {
+			if (didFire) {
+				return;
+			} else if (result) {
 				result.dispose();
 			} else {
-				sync = true;
+				didFire = true;
 			}
 
 			return listener.call(thisArgs, e);
 		}, null, disposables);
 
-		if (sync) {
+		if (didFire) {
 			result.dispose();
 		}