fix(drag-drop): emitting incorrect index for horizontal list in rtl (#13274)

Fixes the wrong index being emitted if the list is horizontal in an RTL layout.

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

@@ -569,6 +569,45 @@ describe('CdkDrag', () => {
           .toEqual(['One', 'Two', 'Zero', 'Three']);
     }));
 
+    it('should dispatch the correct `dropped` event in RTL horizontal drop zone', fakeAsync(() => {
+      const fixture = createComponent(DraggableInHorizontalDropZone, [{
+        provide: Directionality,
+        useValue: ({value: 'rtl'})
+      }]);
+
+      fixture.nativeElement.setAttribute('dir', 'rtl');
+      fixture.detectChanges();
+      const dragItems = fixture.componentInstance.dragItems;
+
+      expect(dragItems.map(drag => drag.element.nativeElement.textContent!.trim()))
+          .toEqual(['Zero', 'One', 'Two', 'Three']);
+
+      const firstItem = dragItems.first;
+      const thirdItemRect = dragItems.toArray()[2].element.nativeElement.getBoundingClientRect();
+
+      dragElementViaMouse(fixture, firstItem.element.nativeElement,
+          thirdItemRect.right - 1, thirdItemRect.top + 1);
+      flush();
+      fixture.detectChanges();
+
+      expect(fixture.componentInstance.droppedSpy).toHaveBeenCalledTimes(1);
+
+      const event = fixture.componentInstance.droppedSpy.calls.mostRecent().args[0];
+
+      // Assert the event like this, rather than `toHaveBeenCalledWith`, because Jasmine will
+      // go into an infinite loop trying to stringify the event, if the test fails.
+      expect(event).toEqual({
+        previousIndex: 0,
+        currentIndex: 2,
+        item: firstItem,
+        container: fixture.componentInstance.dropInstance,
+        previousContainer: fixture.componentInstance.dropInstance
+      });
+
+      expect(dragItems.map(drag => drag.element.nativeElement.textContent!.trim()))
+          .toEqual(['One', 'Two', 'Zero', 'Three']);
+    }));
+
     it('should not move items in a horizontal list if pointer is too far away', fakeAsync(() => {
       const fixture = createComponent(DraggableInHorizontalDropZone);
       fixture.detectChanges();

src/cdk/drag-drop/drop.ts

@@ -20,7 +20,9 @@ import {
   Output,
   QueryList,
   ViewEncapsulation,
+  Optional,
 } from '@angular/core';
+import {Directionality} from '@angular/cdk/bidi';
 import {CdkDrag} from './drag';
 import {DragDropRegistry} from './drag-drop-registry';
 import {CdkDragDrop, CdkDragEnter, CdkDragExit} from './drag-events';
@@ -103,7 +105,8 @@ export class CdkDrop<T = any> implements OnInit, OnDestroy {
 
   constructor(
     public element: ElementRef<HTMLElement>,
-    private _dragDropRegistry: DragDropRegistry<CdkDrag, CdkDrop<T>>) {}
+    private _dragDropRegistry: DragDropRegistry<CdkDrag, CdkDrop<T>>,
+    @Optional() private _dir?: Directionality) {}
 
   ngOnInit() {
     this._dragDropRegistry.registerDropContainer(this);
@@ -217,9 +220,17 @@ export class CdkDrop<T = any> implements OnInit, OnDestroy {
    * @param item Item whose index should be determined.
    */
   getItemIndex(item: CdkDrag): number {
-    return this._dragging ?
-        findIndex(this._positionCache.items, currentItem => currentItem.drag === item) :
-        this._draggables.toArray().indexOf(item);
+    if (!this._dragging) {
+      return this._draggables.toArray().indexOf(item);
+    }
+
+    // Items are sorted always by top/left in the cache, however they flow differently in RTL.
+    // The rest of the logic still stands no matter what orientation we're in, however
+    // we need to invert the array when determining the index.
+    const items = this.orientation === 'horizontal' && this._dir && this._dir.value === 'rtl' ?
+        this._positionCache.items.slice().reverse() : this._positionCache.items;
+
+    return findIndex(items, currentItem => currentItem.drag === item);
   }
 
   /**