fix(drag-drop): account for out of view container and stacking order

Currently we use the `ClientRect` of each drop container to determine whether an item can be dropped into it. Since the `ClientRect` doesn't know whether the container is scrolled out of the view or is positioned under another element, users can end up accidentally moving items into containers that aren't visible.

These changes rework the logic to look for which containers intersect with the user's pointer, and to filter based on whether the container is the top-most element at the pointer's position.

Fixes #14231.

Author: crisbeto

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

@@ -27,6 +27,7 @@ import {moveItemInArray} from './drag-utils';
 import {CdkDropList} from './drop-list';
 import {CdkDragHandle} from './drag-handle';
 import {CdkDropListGroup} from './drop-list-group';
+import {extendStyles} from './drag-styling';
 
 const ITEM_HEIGHT = 25;
 const ITEM_WIDTH = 75;
@@ -989,6 +990,8 @@ describe('CdkDrag', () => {
           .toBe('ltr', 'Expected preview element to inherit the directionality.');
       expect(previewRect.width).toBe(itemRect.width, 'Expected preview width to match element');
       expect(previewRect.height).toBe(itemRect.height, 'Expected preview height to match element');
+      expect(preview.style.pointerEvents)
+          .toBe('none', 'Expected pointer events to be disabled on the preview');
 
       dispatchMouseEvent(document, 'mouseup');
       fixture.detectChanges();
@@ -2227,6 +2230,45 @@ describe('CdkDrag', () => {
       expect(Array.from(component.group._items)).toEqual([component.listOne, component.listTwo]);
     }));
 
+    it('should not be able to drop an element into a container that is under another element',
+      fakeAsync(() => {
+        const fixture = createComponent(ConnectedDropZones);
+        fixture.detectChanges();
+
+        const groups = fixture.componentInstance.groupedDragItems.slice();
+        const element = groups[0][1].element.nativeElement;
+        const dropInstances = fixture.componentInstance.dropInstances.toArray();
+        const targetRect = groups[1][2].element.nativeElement.getBoundingClientRect();
+        const coverElement = document.createElement('div');
+        const targetGroupRect = dropInstances[1].element.nativeElement.getBoundingClientRect();
+
+        // Add an extra element that covers the target container.
+        fixture.nativeElement.appendChild(coverElement);
+        extendStyles(coverElement.style, {
+          position: 'fixed',
+          top: targetGroupRect.top + 'px',
+          left: targetGroupRect.left + 'px',
+          bottom: targetGroupRect.bottom + 'px',
+          right: targetGroupRect.right + 'px',
+          background: 'orange'
+        });
+
+        dragElementViaMouse(fixture, element, targetRect.left + 1, targetRect.top + 1);
+        flush();
+        fixture.detectChanges();
+
+        const event = fixture.componentInstance.droppedSpy.calls.mostRecent().args[0];
+
+        expect(event).toBeTruthy();
+        expect(event).toEqual({
+          previousIndex: 1,
+          currentIndex: 1,
+          item: groups[0][1],
+          container: dropInstances[0],
+          previousContainer: dropInstances[0]
+        });
+      }));
+
   });
 
 });

src/cdk/drag-drop/drag.ts

@@ -638,6 +638,9 @@ export class CdkDrag<T = any> implements AfterViewInit, OnDestroy {
     }
 
     extendStyles(preview.style, {
+      // It's important that we disable the pointer events on the preview, because
+      // it can throw off the `document.elementFromPoint` calls in the `CdkDropList`.
+      pointerEvents: 'none',
       position: 'fixed',
       top: '0',
       left: '0',

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

@@ -21,8 +21,10 @@ import {
   Directive,
   ChangeDetectorRef,
   SkipSelf,
+  Inject,
 } from '@angular/core';
 import {Directionality} from '@angular/cdk/bidi';
+import {DOCUMENT} from '@angular/common';
 import {CdkDrag} from './drag';
 import {DragDropRegistry} from './drag-drop-registry';
 import {CdkDragDrop, CdkDragEnter, CdkDragExit, CdkDragSortEvent} from './drag-events';
@@ -93,6 +95,8 @@ interface ListPositionCacheEntry {
   }
 })
 export class CdkDropList<T = any> implements OnInit, OnDestroy {
+  private _document: Document | undefined;
+
   /** Draggable items in the container. */
   @ContentChildren(forwardRef(() => CdkDrag)) _draggables: QueryList<CdkDrag>;
 
@@ -160,7 +164,17 @@ export class CdkDropList<T = any> implements OnInit, OnDestroy {
     private _dragDropRegistry: DragDropRegistry<CdkDrag, CdkDropList<T>>,
     private _changeDetectorRef: ChangeDetectorRef,
     @Optional() private _dir?: Directionality,
-    @Optional() @SkipSelf() private _group?: CdkDropListGroup<CdkDropList>) {}
+    @Optional() @SkipSelf() private _group?: CdkDropListGroup<CdkDropList>,
+    // @breaking-change 8.0.0 `_document` parameter to be made required.
+    @Optional() @Inject(DOCUMENT) _document?: any) {
+
+    // @breaking-change 8.0.0 Remove null checks once `_document` parameter is required.
+    if (_document) {
+      this._document = _document;
+    } else if (typeof document !== 'undefined') {
+      this._document = document;
+    }
+  }
 
   ngOnInit() {
     this._dragDropRegistry.registerDropContainer(this);
@@ -379,8 +393,39 @@ export class CdkDropList<T = any> implements OnInit, OnDestroy {
    * @param y Position of the item along the Y axis.
    */
   _getSiblingContainerFromPosition(item: CdkDrag, x: number, y: number): CdkDropList | null {
-    const result = this._positionCache.siblings
-        .find(sibling => isInsideClientRect(sibling.clientRect, x, y));
+    const results = this._positionCache.siblings.filter(sibling => {
+      return isInsideClientRect(sibling.clientRect, x, y);
+    });
+
+    // No drop containers are intersecting with the pointer.
+    if (!results.length) {
+      return null;
+    }
+
+    let result: ListPositionCacheEntry | undefined = results[0];
+
+    // @breaking-change 8.0.0 remove null check once the
+    //  `_document` is made into a required parameter.
+    if (this._document) {
+      const elementFromPoint = this._document.elementFromPoint(x, y);
+
+      // If there's no element at the pointer position, then
+      // the client rect is probably scrolled out of the view.
+      if (!elementFromPoint) {
+        return null;
+      }
+
+      // The `ClientRect`, that we're using to find the container over which the user is
+      // hovering, doesn't give us any information on whether the element has been scrolled
+      // out of the view or whether it's overlapping with other containers. This means that
+      // we could end up transferring the item into a container that's invisible or is positioned
+      // below another one. We use the result from `elementFromPoint` to get the top-most element
+      // at the pointer position and to find whether it's one of the intersecting drop containers.
+      result = results.find(sibling => {
+        const element = sibling.drop.element.nativeElement;
+        return element === elementFromPoint || element.contains(elementFromPoint);
+      });
+    }
 
     return result && result.drop.enterPredicate(item, result.drop) ? result.drop : null;
   }

tools/public_api_guard/cdk/drag-drop.d.ts

@@ -118,7 +118,7 @@ export declare class CdkDropList<T = any> implements OnInit, OnDestroy {
     lockAxis: 'x' | 'y';
     orientation: 'horizontal' | 'vertical';
     sorted: EventEmitter<CdkDragSortEvent<T>>;
-    constructor(element: ElementRef<HTMLElement>, _dragDropRegistry: DragDropRegistry<CdkDrag, CdkDropList<T>>, _changeDetectorRef: ChangeDetectorRef, _dir?: Directionality | undefined, _group?: CdkDropListGroup<CdkDropList<any>> | undefined);
+    constructor(element: ElementRef<HTMLElement>, _dragDropRegistry: DragDropRegistry<CdkDrag, CdkDropList<T>>, _changeDetectorRef: ChangeDetectorRef, _dir?: Directionality | undefined, _group?: CdkDropListGroup<CdkDropList<any>> | undefined, _document?: any);
     _canReturnItem(x: number, y: number): boolean;
     _getSiblingContainerFromPosition(item: CdkDrag, x: number, y: number): CdkDropList | null;
     _sortItem(item: CdkDrag, pointerX: number, pointerY: number, pointerDelta: {