fix(cdk/drag-drop): stop pointer events on placeholder (#24404)

Removes the pointer events from the drag placeholder so that the user can't interact with it while the preview is animating.

Fixes #24403.

Author: crisbeto

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

@@ -259,6 +259,18 @@ describe('CdkDrag', () => {
 
         expect(dragElement.style.transform).toBeFalsy();
       }));
+
+      it('should prevent the default dragstart action', fakeAsync(() => {
+        const fixture = createComponent(StandaloneDraggable);
+        fixture.detectChanges();
+        const event = dispatchFakeEvent(
+          fixture.componentInstance.dragElement.nativeElement,
+          'dragstart',
+        );
+        fixture.detectChanges();
+
+        expect(event.defaultPrevented).toBe(true);
+      }));
     });
 
     describe('touch dragging', () => {
@@ -2952,6 +2964,9 @@ describe('CdkDrag', () => {
       expect(placeholder.textContent!.trim())
         .withContext('Expected placeholder content to match element')
         .toContain('One');
+      expect(placeholder.style.pointerEvents)
+        .withContext('Expected pointer events to be disabled on placeholder')
+        .toBe('none');
 
       dispatchMouseEvent(document, 'mouseup');
       fixture.detectChanges();

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

@@ -442,6 +442,9 @@ export class DragRef<T = any> {
       this._ngZone.runOutsideAngular(() => {
         element.addEventListener('mousedown', this._pointerDown, activeEventListenerOptions);
         element.addEventListener('touchstart', this._pointerDown, passiveEventListenerOptions);
+        // Usually this isn't necessary since the we prevent the default action in `pointerDown`,
+        // but some cases like dragging of links can slip through (see #24403).
+        element.addEventListener('dragstart', preventDefault, activeEventListenerOptions);
       });
       this._initialTransform = undefined;
       this._rootElement = element;
@@ -1159,6 +1162,9 @@ export class DragRef<T = any> {
       placeholder = deepCloneNode(this._rootElement);
     }
 
+    // Stop pointer events on the preview so the user can't
+    // interact with it while the preview is animating.
+    placeholder.style.pointerEvents = 'none';
     placeholder.classList.add('cdk-drag-placeholder');
     return placeholder;
   }
@@ -1290,6 +1296,7 @@ export class DragRef<T = any> {
   private _removeRootElementListeners(element: HTMLElement) {
     element.removeEventListener('mousedown', this._pointerDown, activeEventListenerOptions);
     element.removeEventListener('touchstart', this._pointerDown, passiveEventListenerOptions);
+    element.removeEventListener('dragstart', preventDefault, activeEventListenerOptions);
   }
 
   /**
@@ -1555,3 +1562,8 @@ function matchElementSize(target: HTMLElement, sourceRect: ClientRect): void {
   target.style.height = `${sourceRect.height}px`;
   target.style.transform = getTransform(sourceRect.left, sourceRect.top);
 }
+
+/** Utility to prevent the default action of an event. */
+function preventDefault(event: Event): void {
+  event.preventDefault();
+}