fix(cdk/a11y): detect fake touchstart events from screen readers

We currently have handling for the case where a `mousedown` event is thrown off by a
fake `mousedown` listener that may be dispatched by a screen reader when an element
is activated. It turns out that if the device has touch support, screen readers may dispatch
a fake `touchstart` event instead of a fake `mousedown`.

These changes add another utility function that allows us to distinguish the fake events
and fix some issues where keyboard focus wasn't being shown because of the fake
`touchstart` events.

Fixes #21947.

src/cdk/a11y/fake-event-detection.ts

@@ -0,0 +1,29 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+/** Gets whether an event could be a faked `mousedown` event dispatched by a screen reader. */
+export function isFakeMousedownFromScreenReader(event: MouseEvent): boolean {
+  // We can typically distinguish between these faked mousedown events and real mousedown events
+  // using the "buttons" property. While real mousedowns will indicate the mouse button that was
+  // pressed (e.g. "1" for the left mouse button), faked mousedowns will usually set the property
+  // value to 0.
+  return event.buttons === 0;
+}
+
+/** Gets whether an event could be a faked `touchstart` event dispatched by a screen reader. */
+export function isFakeTouchstartFromScreenReader(event: TouchEvent): boolean {
+  const touch: Touch | undefined = (event.touches && event.touches[0]) ||
+                                   (event.changedTouches && event.changedTouches[0]);
+
+  // A fake `touchstart` can be distinguished from a real one by looking at the `identifier`
+  // which is typically >= 0 on a real device versus -1 from a screen reader. Just to be safe,
+  // we can also look at `radiusX` and `radiusY`. This behavior was observed against a Windows 10
+  // device with a touch screen running NVDA v2020.4 and Firefox 85 or Chrome 88.
+  return !!touch && touch.identifier === -1 && (touch.radiusX == null || touch.radiusX === 1) &&
+         (touch.radiusY == null || touch.radiusY === 1);
+}

src/cdk/a11y/focus-monitor/focus-monitor.ts

@@ -23,7 +23,10 @@ import {
 import {Observable, of as observableOf, Subject, Subscription} from 'rxjs';
 import {coerceElement} from '@angular/cdk/coercion';
 import {DOCUMENT} from '@angular/common';
-import {isFakeMousedownFromScreenReader} from '../fake-mousedown';
+import {
+  isFakeMousedownFromScreenReader,
+  isFakeTouchstartFromScreenReader,
+} from '../fake-event-detection';
 
 
 // This is the value used by AngularJS Material. Through trial and error (on iPhone 6S) they found
@@ -156,15 +159,21 @@ export class FocusMonitor implements OnDestroy {
    * Needs to be an arrow function in order to preserve the context when it gets bound.
    */
   private _documentTouchstartListener = (event: TouchEvent) => {
-    // When the touchstart event fires the focus event is not yet in the event queue. This means
-    // we can't rely on the trick used above (setting timeout of 1ms). Instead we wait 650ms to
-    // see if a focus happens.
-    if (this._touchTimeoutId != null) {
-      clearTimeout(this._touchTimeoutId);
-    }
+    // Some screen readers will fire a fake `touchstart` event if an element is activated using
+    // the keyboard while on a device with a touchsreen. Consider such events as keyboard focus.
+    if (!isFakeTouchstartFromScreenReader(event)) {
+      // When the touchstart event fires the focus event is not yet in the event queue. This means
+      // we can't rely on the trick used above (setting timeout of 1ms). Instead we wait 650ms to
+      // see if a focus happens.
+      if (this._touchTimeoutId != null) {
+        clearTimeout(this._touchTimeoutId);
+      }
 
-    this._lastTouchTarget = getTarget(event);
-    this._touchTimeoutId = setTimeout(() => this._lastTouchTarget = null, TOUCH_BUFFER_MS);
+      this._lastTouchTarget = getTarget(event);
+      this._touchTimeoutId = setTimeout(() => this._lastTouchTarget = null, TOUCH_BUFFER_MS);
+    } else if (!this._lastTouchTarget) {
+      this._setOriginForCurrentEventQueue('keyboard');
+    }
   }
 
   /**

src/cdk/a11y/public-api.ts

@@ -18,7 +18,7 @@ export * from './interactivity-checker/interactivity-checker';
 export * from './live-announcer/live-announcer';
 export * from './live-announcer/live-announcer-tokens';
 export * from './focus-monitor/focus-monitor';
-export * from './fake-mousedown';
+export * from './fake-event-detection';
 export * from './a11y-module';
 export {
   HighContrastModeDetector,

src/material/core/ripple/ripple-renderer.ts

@@ -7,7 +7,7 @@
  */
 import {ElementRef, NgZone} from '@angular/core';
 import {Platform, normalizePassiveListenerOptions} from '@angular/cdk/platform';
-import {isFakeMousedownFromScreenReader} from '@angular/cdk/a11y';
+import {isFakeMousedownFromScreenReader, isFakeTouchstartFromScreenReader} from '@angular/cdk/a11y';
 import {coerceElement} from '@angular/cdk/coercion';
 import {RippleRef, RippleState, RippleConfig} from './ripple-ref';
 
@@ -259,7 +259,7 @@ export class RippleRenderer implements EventListenerObject {
 
   /** Function being called whenever the trigger is being pressed using touch. */
   private _onTouchStart(event: TouchEvent) {
-    if (!this._target.rippleDisabled) {
+    if (!this._target.rippleDisabled && !isFakeTouchstartFromScreenReader(event)) {
       // Some browsers fire mouse events after a `touchstart` event. Those synthetic mouse
       // events will launch a second ripple if we don't ignore mouse events for a specific
       // time after a touchstart event.

src/material/menu/menu-trigger.ts

@@ -6,7 +6,12 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {FocusMonitor, FocusOrigin, isFakeMousedownFromScreenReader} from '@angular/cdk/a11y';
+import {
+  FocusMonitor,
+  FocusOrigin,
+  isFakeMousedownFromScreenReader,
+  isFakeTouchstartFromScreenReader,
+} from '@angular/cdk/a11y';
 import {Direction, Directionality} from '@angular/cdk/bidi';
 import {ENTER, LEFT_ARROW, RIGHT_ARROW, SPACE} from '@angular/cdk/keycodes';
 import {
@@ -99,7 +104,11 @@ export class MatMenuTrigger implements AfterContentInit, OnDestroy {
    * Handles touch start events on the trigger.
    * Needs to be an arrow function so we can easily use addEventListener and removeEventListener.
    */
-  private _handleTouchStart = () => this._openedBy = 'touch';
+  private _handleTouchStart = (event: TouchEvent) => {
+    if (!isFakeTouchstartFromScreenReader(event)) {
+      this._openedBy = 'touch';
+    }
+  }
 
   // Tracking input type is necessary so it's possible to only auto-focus
   // the first item of the list when the menu is opened via the keyboard

tools/public_api_guard/cdk/a11y.d.ts

@@ -193,6 +193,8 @@ export declare class InteractivityChecker {
 
 export declare function isFakeMousedownFromScreenReader(event: MouseEvent): boolean;
 
+export declare function isFakeTouchstartFromScreenReader(event: TouchEvent): boolean;
+
 export declare class IsFocusableConfig {
     ignoreVisibility: boolean;
 }