fix(cdk/drag-drop): don't start dragging on fake screen reader events (#23126)

Fixes that the CDK drag&drop could be triggered by fake `mousedown` or `touchstart` events dispatched by a screen reader.

Author: crisbeto

src/cdk/drag-drop/BUILD.bazel

@@ -17,6 +17,7 @@ ng_module(
     module_name = "@angular/cdk/drag-drop",
     deps = [
         "//src:dev_mode_types",
+        "//src/cdk/a11y",
         "//src/cdk/bidi",
         "//src/cdk/coercion",
         "//src/cdk/platform",

src/cdk/drag-drop/directives/drag.spec.ts

@@ -253,6 +253,33 @@ describe('CdkDrag', () => {
         expect(mousedownEvent.preventDefault).toHaveBeenCalled();
       }));
 
+      it('should not start dragging an element with a fake mousedown event', fakeAsync(() => {
+        const fixture = createComponent(StandaloneDraggable);
+        fixture.detectChanges();
+        const dragElement = fixture.componentInstance.dragElement.nativeElement;
+        const event = createMouseEvent('mousedown', 0, 0);
+
+        Object.defineProperties(event, {
+          offsetX: {get: () => 0},
+          offsetY: {get: () => 0}
+        });
+
+        expect(dragElement.style.transform).toBeFalsy();
+
+        dispatchEvent(dragElement, event);
+        fixture.detectChanges();
+
+        dispatchMouseEvent(document, 'mousemove', 20, 100);
+        fixture.detectChanges();
+        dispatchMouseEvent(document, 'mousemove', 50, 100);
+        fixture.detectChanges();
+
+        dispatchMouseEvent(document, 'mouseup');
+        fixture.detectChanges();
+
+        expect(dragElement.style.transform).toBeFalsy();
+      }));
+
     });
 
     describe('touch dragging', () => {
@@ -350,6 +377,34 @@ describe('CdkDrag', () => {
 
         expect(touchstartEvent.preventDefault).not.toHaveBeenCalled();
       }));
+
+      it('should not start dragging an element with a fake touchstart event', fakeAsync(() => {
+        const fixture = createComponent(StandaloneDraggable);
+        fixture.detectChanges();
+        const dragElement = fixture.componentInstance.dragElement.nativeElement;
+        const event = createTouchEvent('touchstart', 50, 50) as TouchEvent;
+
+        Object.defineProperties(event.touches[0], {
+          identifier: {get: () => -1},
+          radiusX: {get: () => null},
+          radiusY: {get: () => null}
+        });
+
+        expect(dragElement.style.transform).toBeFalsy();
+
+        dispatchEvent(dragElement, event);
+        fixture.detectChanges();
+
+        dispatchTouchEvent(document, 'touchmove', 20, 100);
+        fixture.detectChanges();
+        dispatchTouchEvent(document, 'touchmove', 50, 100);
+        fixture.detectChanges();
+
+        dispatchTouchEvent(document, 'touchend');
+        fixture.detectChanges();
+
+        expect(dragElement.style.transform).toBeFalsy();
+      }));
     });
 
     it('should dispatch an event when the user has started dragging', fakeAsync(() => {

src/cdk/drag-drop/drag-ref.ts

@@ -15,6 +15,10 @@ import {
   _getShadowRoot,
 } from '@angular/cdk/platform';
 import {coerceBooleanProperty, coerceElement} from '@angular/cdk/coercion';
+import {
+  isFakeMousedownFromScreenReader,
+  isFakeTouchstartFromScreenReader,
+} from '@angular/cdk/a11y';
 import {Subscription, Subject, Observable} from 'rxjs';
 import {DropListRefInternal as DropListRef} from './drop-list-ref';
 import {DragDropRegistry} from './drag-drop-registry';
@@ -864,6 +868,8 @@ export class DragRef<T = any> {
     const target = _getEventTarget(event);
     const isSyntheticEvent = !isTouchSequence && this._lastTouchEventTime &&
       this._lastTouchEventTime + MOUSE_EVENT_IGNORE_TIME > Date.now();
+    const isFakeEvent = isTouchSequence ? isFakeTouchstartFromScreenReader(event as TouchEvent) :
+      isFakeMousedownFromScreenReader(event as MouseEvent);
 
     // If the event started from an element with the native HTML drag&drop, it'll interfere
     // with our own dragging (e.g. `img` tags do it by default). Prevent the default action
@@ -876,7 +882,7 @@ export class DragRef<T = any> {
     }
 
     // Abort if the user is already dragging or is using a mouse button other than the primary one.
-    if (isDragging || isAuxiliaryMouseButton || isSyntheticEvent) {
+    if (isDragging || isAuxiliaryMouseButton || isSyntheticEvent || isFakeEvent) {
       return;
     }