Prevent undefined being fired when a listener is disposed

src/vs/base/common/event.ts

@@ -254,7 +254,7 @@ export namespace Event {
 				});
 			},
 			onWillRemoveListener() {
-				if (flushOnListenerRemove) {
+				if (flushOnListenerRemove && numDebouncedCalls > 0) {
 					doFire?.();
 				}
 			},

src/vs/base/test/common/event.test.ts

@@ -1048,6 +1048,26 @@ suite('Event utils', () => {
 	});
 
 	suite('accumulate', () => {
+		test('should not fire after a listener is disposed with undefined or []', async () => {
+			const eventEmitter = new Emitter<number>();
+			const event = eventEmitter.event;
+			const accumulated = Event.accumulate(event, 0);
+
+			const calls1: number[][] = [];
+			const calls2: number[][] = [];
+			const listener1 = accumulated((e) => calls1.push(e));
+			accumulated((e) => calls2.push(e));
+
+			eventEmitter.fire(1);
+			await timeout(1);
+			assert.deepStrictEqual(calls1, [[1]]);
+			assert.deepStrictEqual(calls2, [[1]]);
+
+			listener1.dispose();
+			await timeout(1);
+			assert.deepStrictEqual(calls1, [[1]]);
+			assert.deepStrictEqual(calls2, [[1]], 'should not fire after a listener is disposed with undefined or []');
+		});
 		test('should accumulate a single event', async () => {
 			const eventEmitter = new Emitter<number>();
 			const event = eventEmitter.event;
@@ -1234,7 +1254,6 @@ suite('Event utils', () => {
 			assert.deepStrictEqual(calls, [1], 'should fire with the first event, not the second (after listener dispose)');
 		});
 
-
 		test('should flush events when the emitter is disposed', async () => {
 			const emitter = new Emitter<number>();
 			const debounced = Event.debounce(emitter.event, (l, e) => l ? l + 1 : 1, 0);