Fix tab close button getting stuck on "Close Other Editors" after Alt+Tab

src/vs/base/browser/dom.ts

@@ -1854,12 +1854,14 @@ export class ModifierKeyEmitter extends event.Emitter<IModifierKeyStatus> {
 				return;
 			}
 
+			const modifiersChanged = this.hasModifierChanges(e);
+
 			this._keyStatus.altKey = e.altKey;
 			this._keyStatus.ctrlKey = e.ctrlKey;
 			this._keyStatus.metaKey = e.metaKey;
 			this._keyStatus.shiftKey = e.shiftKey;
 
-			if (this._keyStatus.lastKeyPressed) {
+			if (this._keyStatus.lastKeyPressed || modifiersChanged) {
 				this._keyStatus.event = e;
 				this.fire(this._keyStatus);
 			}
@@ -1886,36 +1888,74 @@ export class ModifierKeyEmitter extends event.Emitter<IModifierKeyStatus> {
 				this._keyStatus.lastKeyPressed = undefined;
 			}
 
+			const modifiersChanged = this.hasModifierChanges(e);
+
 			this._keyStatus.altKey = e.altKey;
 			this._keyStatus.ctrlKey = e.ctrlKey;
 			this._keyStatus.metaKey = e.metaKey;
 			this._keyStatus.shiftKey = e.shiftKey;
 
-			if (this._keyStatus.lastKeyReleased) {
+			if (this._keyStatus.lastKeyReleased || modifiersChanged) {
 				this._keyStatus.event = e;
 				this.fire(this._keyStatus);
 			}
 		}, true));
 
-		disposables.add(addDisposableListener(window.document.body, 'mousedown', () => {
+		disposables.add(addDisposableListener(window.document.body, 'mousedown', e => {
 			this._keyStatus.lastKeyPressed = undefined;
+
+			this.syncKeyStatus(e);
 		}, true));
 
-		disposables.add(addDisposableListener(window.document.body, 'mouseup', () => {
+		disposables.add(addDisposableListener(window.document.body, 'mouseup', e => {
 			this._keyStatus.lastKeyPressed = undefined;
+
+			this.syncKeyStatus(e);
 		}, true));
 
 		disposables.add(addDisposableListener(window.document.body, 'mousemove', e => {
 			if (e.buttons) {
 				this._keyStatus.lastKeyPressed = undefined;
 			}
+
+			this.syncKeyStatus(e);
 		}, true));
 
 		disposables.add(addDisposableListener(window, 'blur', () => {
 			this.resetKeyStatus();
 		}));
 	}
 
+	/**
+	 * Modifier state goes stale when a modifier is released while our windows are not
+	 * focused (e.g. during Alt+Tab or macOS Mission Control) because the matching
+	 * `keyup` never arrives. Mouse events carry the actual modifier state, so they
+	 * allow to recover from that (#331979).
+	 */
+	private syncKeyStatus(e: MouseEvent): void {
+		if (!this.hasModifierChanges(e)) {
+			return;
+		}
+
+		// The keyboard transition we missed makes the last pressed and
+		// released keys unreliable, so continue with a clean status.
+		this._keyStatus = {
+			altKey: e.altKey,
+			ctrlKey: e.ctrlKey,
+			metaKey: e.metaKey,
+			shiftKey: e.shiftKey
+		};
+
+		this.fire(this._keyStatus);
+	}
+
+	private hasModifierChanges(e: KeyboardEvent | MouseEvent): boolean {
+		return this._keyStatus.altKey !== e.altKey ||
+			this._keyStatus.ctrlKey !== e.ctrlKey ||
+			this._keyStatus.metaKey !== e.metaKey ||
+			this._keyStatus.shiftKey !== e.shiftKey;
+	}
+
 	get keyStatus(): IModifierKeyStatus {
 		return this._keyStatus;
 	}

src/vs/base/test/browser/dom.test.ts

@@ -4,11 +4,12 @@
  *--------------------------------------------------------------------------------------------*/
 
 import assert from 'assert';
-import { $, h, trackAttributes, copyAttributes, disposableWindowInterval, getWindows, getWindowsCount, getWindowId, getWindowById, hasWindow, getWindow, getDocument, isHTMLElement, SafeTriangle, AnimationFrameScheduler, DisposableResizeObserver, getRecentDisposableResizeObserverContextForLoopError, findParentWithClass, hasParentWithClass } from '../../browser/dom.js';
+import { $, h, trackAttributes, copyAttributes, disposableWindowInterval, getWindows, getWindowsCount, getWindowId, getWindowById, hasWindow, getWindow, getDocument, isHTMLElement, SafeTriangle, AnimationFrameScheduler, DisposableResizeObserver, getRecentDisposableResizeObserverContextForLoopError, findParentWithClass, hasParentWithClass, ModifierKeyEmitter } from '../../browser/dom.js';
 import { asCssValueWithDefault } from '../../../base/browser/cssValue.js';
 import { ensureCodeWindow, isAuxiliaryWindow, mainWindow } from '../../browser/window.js';
 import { DeferredPromise, timeout } from '../../common/async.js';
 import { errorHandler, setUnexpectedErrorHandler } from '../../common/errors.js';
+import { DisposableStore } from '../../common/lifecycle.js';
 import { runWithFakedTimers } from '../common/timeTravelScheduler.js';
 import { ensureNoDisposablesAreLeakedInTestSuite } from '../common/utils.js';
 
@@ -737,5 +738,48 @@ suite('dom', () => {
 		});
 	});
 
+	suite('ModifierKeyEmitter', () => {
+		const disposables = new DisposableStore();
+
+		teardown(() => {
+			disposables.clear();
+			ModifierKeyEmitter.disposeInstance();
+		});
+
+		function trackAltKey(emitter: ModifierKeyEmitter): boolean[] {
+			const altKeyUpdates: boolean[] = [];
+			disposables.add(emitter.event(status => altKeyUpdates.push(status.altKey)));
+
+			return altKeyUpdates;
+		}
+
+		test('notifies when a keyboard event reveals that Alt is no longer pressed', () => {
+			const emitter = ModifierKeyEmitter.getInstance();
+			const altKeyUpdates = trackAltKey(emitter);
+
+			mainWindow.dispatchEvent(new KeyboardEvent('keydown', { key: 'Alt', altKey: true }));
+
+			// Alt got released while another application had focus, so the matching
+			// `keyup` never arrived and the next keyboard event reveals the truth
+			mainWindow.dispatchEvent(new KeyboardEvent('keydown', { key: 'a' }));
+
+			assert.deepStrictEqual({ altKeyUpdates, altKey: emitter.keyStatus.altKey }, { altKeyUpdates: [true, false], altKey: false });
+		});
+
+		test('mouse events keep the modifier state in sync', () => {
+			const emitter = ModifierKeyEmitter.getInstance();
+			const altKeyUpdates = trackAltKey(emitter);
+
+			mainWindow.dispatchEvent(new KeyboardEvent('keydown', { key: 'Alt', altKey: true }));
+
+			// Mouse events carry the actual modifier state and are the only
+			// signal we get when the `keyup` for Alt was never delivered
+			mainWindow.document.body.dispatchEvent(new MouseEvent('mousemove'));
+			mainWindow.document.body.dispatchEvent(new MouseEvent('mousedown'));
+
+			assert.deepStrictEqual({ altKeyUpdates, altKey: emitter.keyStatus.altKey }, { altKeyUpdates: [true, false], altKey: false });
+		});
+	});
+
 	ensureNoDisposablesAreLeakedInTestSuite();
 });