Fix #29642 - debounced event fired multiple times for a single call with `leading = true`

src/vs/base/common/event.ts

@@ -315,10 +315,13 @@ export function debounceEvent<I, O>(event: Event<I>, merger: (last: O, event: I)
 	let subscription: IDisposable;
 	let output: O;
 	let handle: number;
+	let numDebouncedCalls = 0;
 
 	const emitter = new Emitter<O>({
 		onFirstListenerAdd() {
 			subscription = event(cur => {
+				numDebouncedCalls++;
+
 				output = merger(output, cur);
 				if (!handle && leading) {
 					emitter.fire(output);
@@ -328,8 +331,12 @@ export function debounceEvent<I, O>(event: Event<I>, merger: (last: O, event: I)
 				handle = setTimeout(() => {
 					let _output = output;
 					output = undefined;
-					emitter.fire(_output);
+					if (!leading || numDebouncedCalls > 1) {
+						emitter.fire(_output);
+					}
+
 					handle = null;
+					numDebouncedCalls = 0;
 				}, delay);
 			});
 		},

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

@@ -209,6 +209,42 @@ suite('Event', function () {
 		doc.setText('2');
 		doc.setText('3');
 	});
+
+	test('Debounce Event - leading', function (done: () => void) {
+		const emitter = new Emitter<void>();
+		let debounced = debounceEvent(emitter.event, (l, e) => e, 0, /*leading=*/true);
+
+		let calls = 0;
+		debounced(() => {
+			calls++;
+		});
+
+		// If the source event is fired once, the debounced (on the leading edge) event should be fired only once
+		emitter.fire();
+		setTimeout(() => {
+			assert.equal(calls, 1);
+			done();
+		});
+	});
+
+	test('Debounce Event - leading', function (done: () => void) {
+		const emitter = new Emitter<void>();
+		let debounced = debounceEvent(emitter.event, (l, e) => e, 0, /*leading=*/true);
+
+		let calls = 0;
+		debounced(() => {
+			calls++;
+		});
+
+		// If the source event is fired multiple times, the debounced (on the leading edge) event should be fired twice
+		emitter.fire();
+		emitter.fire();
+		emitter.fire();
+		setTimeout(() => {
+			assert.equal(calls, 2);
+			done();
+		});
+	});
 });
 
 suite('Event utils', () => {