diff --git a/src/vs/base/browser/dom.ts b/src/vs/base/browser/dom.ts
index 2fc731bd888..f4a8b2a9c50 100644
--- a/src/vs/base/browser/dom.ts
+++ b/src/vs/base/browser/dom.ts
@@ -1441,6 +1441,9 @@ export class ModifierKeyEmitter extends Emitter<IModifierKeyStatus> {
 		};
 
 		this._subscriptions.add(domEvent(window, 'keydown', true)(e => {
+			if (e.defaultPrevented) {
+				return;
+			}
 
 			const event = new StandardKeyboardEvent(e);
 			// If Alt-key keydown event is repeated, ignore it #112347
@@ -1475,6 +1478,10 @@ export class ModifierKeyEmitter extends Emitter<IModifierKeyStatus> {
 		}));
 
 		this._subscriptions.add(domEvent(window, 'keyup', true)(e => {
+			if (e.defaultPrevented) {
+				return;
+			}
+
 			if (!e.altKey && this._keyStatus.altKey) {
 				this._keyStatus.lastKeyReleased = 'alt';
 			} else if (!e.ctrlKey && this._keyStatus.ctrlKey) {
diff --git a/src/vs/base/browser/ui/dialog/dialog.ts b/src/vs/base/browser/ui/dialog/dialog.ts
index 2c5d315a82b..c6d17d5cec9 100644
--- a/src/vs/base/browser/ui/dialog/dialog.ts
+++ b/src/vs/base/browser/ui/dialog/dialog.ts
@@ -218,6 +218,10 @@ export class Dialog extends Disposable {
 			this._register(domEvent(window, 'keydown', true)((e: KeyboardEvent) => {
 				const evt = new StandardKeyboardEvent(e);
 
+				if (evt.equals(KeyMod.Alt)) {
+					evt.preventDefault();
+				}
+
 				if (evt.equals(KeyCode.Enter)) {
 
 					// Enter in input field should OK the dialog