fix(cdk/drag-drop): not blocking initial move event

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

@@ -318,8 +318,10 @@ describe('CdkDrag', () => {
           dispatchTouchEvent(fixture.componentInstance.dragElement.nativeElement, 'touchstart');
           fixture.detectChanges();
 
-          dispatchTouchEvent(document, 'touchmove');
-          expect(dispatchTouchEvent(document, 'touchmove').defaultPrevented).toBe(true);
+          expect(dispatchTouchEvent(document, 'touchmove').defaultPrevented)
+              .toBe(true, 'Expected initial touchmove to be prevented.');
+          expect(dispatchTouchEvent(document, 'touchmove').defaultPrevented)
+              .toBe(true, 'Expected subsequent touchmose to be prevented.');
 
           dispatchTouchEvent(document, 'touchend');
           fixture.detectChanges();

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

@@ -629,6 +629,9 @@ export class DragRef<T = any> {
         // being dragged. This can happen while we're waiting for the drop animation to finish
         // and can cause errors, because some elements might still be moving around.
         if (!container || (!container.isDragging() && !container.isReceiving())) {
+          // Prevent the default action as soon as the dragging sequence is considered as
+          // "started" since waiting for the next event can allow the device to begin scrolling.
+          event.preventDefault();
           this._hasStartedDragging = true;
           this._ngZone.run(() => this._startDragSequence(event));
         }