fix(drag-drop): avoid interference from native drag&drop (#14054)

Currently if the user starts dragging from an element that has the native drag&drop (e.g. `img` tags have it by default), the CDK dragging won't work correctly. These changes add a `preventDefault` to work around it.

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

@@ -18,6 +18,7 @@ import {
   dispatchEvent,
   dispatchMouseEvent,
   dispatchTouchEvent,
+  createTouchEvent,
 } from '@angular/cdk/testing';
 import {Directionality} from '@angular/cdk/bidi';
 import {CdkDrag, CDK_DRAG_CONFIG, CdkDragConfig} from './drag';
@@ -169,6 +170,25 @@ describe('CdkDrag', () => {
         expect(dragElement.style.transform).toBe('translateY(-50%) translate3d(150px, 300px, 0px)');
       }));
 
+      it('should prevent the `mousedown` action for native draggable elements', fakeAsync(() => {
+        const fixture = createComponent(StandaloneDraggable);
+        fixture.detectChanges();
+        const dragElement = fixture.componentInstance.dragElement.nativeElement;
+
+        dragElement.draggable = true;
+
+        const mousedownEvent = createMouseEvent('mousedown', 50, 50);
+        Object.defineProperty(mousedownEvent, 'target', {get: () => dragElement});
+        spyOn(mousedownEvent, 'preventDefault').and.callThrough();
+        dispatchEvent(dragElement, mousedownEvent);
+        fixture.detectChanges();
+
+        dispatchMouseEvent(document, 'mousemove', 50, 50);
+        fixture.detectChanges();
+
+        expect(mousedownEvent.preventDefault).toHaveBeenCalled();
+      }));
+
     });
 
     describe('touch dragging', () => {
@@ -244,6 +264,25 @@ describe('CdkDrag', () => {
           dispatchTouchEvent(document, 'touchend');
           fixture.detectChanges();
         }));
+
+      it('should not prevent `touchstart` action for native draggable elements', fakeAsync(() => {
+        const fixture = createComponent(StandaloneDraggable);
+        fixture.detectChanges();
+        const dragElement = fixture.componentInstance.dragElement.nativeElement;
+
+        dragElement.draggable = true;
+
+        const touchstartEvent = createTouchEvent('touchstart', 50, 50);
+        Object.defineProperty(touchstartEvent, 'target', {get: () => dragElement});
+        spyOn(touchstartEvent, 'preventDefault').and.callThrough();
+        dispatchEvent(dragElement, touchstartEvent);
+        fixture.detectChanges();
+
+        dispatchTouchEvent(document, 'touchmove');
+        fixture.detectChanges();
+
+        expect(touchstartEvent.preventDefault).not.toHaveBeenCalled();
+      }));
     });
 
     it('should dispatch an event when the user has started dragging', fakeAsync(() => {

src/cdk/drag-drop/drag.ts

@@ -82,6 +82,9 @@ export function CDK_DRAG_CONFIG_FACTORY(): CdkDragConfig {
 /** Options that can be used to bind a passive event listener. */
 const passiveEventListenerOptions = normalizePassiveListenerOptions({passive: true});
 
+/** Options that can be used to bind an active event listener. */
+const activeEventListenerOptions = normalizePassiveListenerOptions({passive: false});
+
 /** Element that can be moved inside a CdkDropList container. */
 @Directive({
   selector: '[cdkDrag]',
@@ -279,7 +282,7 @@ export class CdkDrag<T = any> implements AfterViewInit, OnDestroy {
       .pipe(take(1))
       .subscribe(() => {
         const rootElement = this._rootElement = this._getRootElement();
-        rootElement.addEventListener('mousedown', this._pointerDown, passiveEventListenerOptions);
+        rootElement.addEventListener('mousedown', this._pointerDown, activeEventListenerOptions);
         rootElement.addEventListener('touchstart', this._pointerDown, passiveEventListenerOptions);
         this._handles.changes.pipe(startWith(null)).subscribe(() =>
             toggleNativeDragInteractions(rootElement, this.getChildHandles().length > 0));
@@ -290,7 +293,7 @@ export class CdkDrag<T = any> implements AfterViewInit, OnDestroy {
     // The directive might have been destroyed before the root element is initialized.
     if (this._rootElement) {
       this._rootElement.removeEventListener('mousedown', this._pointerDown,
-          passiveEventListenerOptions);
+          activeEventListenerOptions);
       this._rootElement.removeEventListener('touchstart', this._pointerDown,
           passiveEventListenerOptions);
 
@@ -354,6 +357,16 @@ export class CdkDrag<T = any> implements AfterViewInit, OnDestroy {
     // starting another sequence for a draggable parent somewhere up the DOM tree.
     event.stopPropagation();
 
+    // 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
+    // to stop it from happening. Note that preventing on `dragstart` also seems to work, but
+    // it's flaky and it fails if the user drags it away quickly. Also note that we only want
+    // to do this for `mousedown` since doing the same for `touchstart` will stop any `click`
+    // events from firing on touch devices.
+    if (event.target && (event.target as HTMLElement).draggable && event.type === 'mousedown') {
+      event.preventDefault();
+    }
+
     // Abort if the user is already dragging or is using a mouse button other than the primary one.
     if (this._isDragging() || (!this._isTouchEvent(event) && event.button !== 0)) {
       return;