fix(cdk/drag-drop): sorted event emitted multiple times for single-item list (#23589)

Fixes that the `sorted` event was being emitted when the item wasn't actually being sorted when it's inside of a list with a single item.

Fixes #23575.

Author: crisbeto

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

@@ -1861,6 +1861,29 @@ describe('CdkDrag', () => {
       flush();
     }));
 
+    it('should not dispatch the `sorted` event when an item is dragged inside ' +
+        'a single-item list', fakeAsync(() => {
+        const fixture = createComponent(DraggableInDropZone);
+        fixture.componentInstance.items = [fixture.componentInstance.items[0]];
+        fixture.detectChanges();
+
+        const draggedItem = fixture.componentInstance.dragItems.first.element.nativeElement;
+        const {top, left} = draggedItem.getBoundingClientRect();
+
+        startDraggingViaMouse(fixture, draggedItem, left, top);
+
+        for (let i = 0; i < 5; i++) {
+          dispatchMouseEvent(document, 'mousemove', left, top + 1);
+          fixture.detectChanges();
+
+          expect(fixture.componentInstance.sortedSpy).not.toHaveBeenCalled();
+        }
+
+        dispatchMouseEvent(document, 'mouseup');
+        fixture.detectChanges();
+        flush();
+      }));
+
     it('should not move items in a vertical list if the pointer is too far away', fakeAsync(() => {
       const fixture = createComponent(DraggableInDropZone);
       fixture.detectChanges();

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

@@ -754,11 +754,10 @@ export class DropListRef<T = any> {
   private _getItemIndexFromPointerPosition(item: DragRef, pointerX: number, pointerY: number,
                                            delta?: {x: number, y: number}): number {
     const isHorizontal = this._orientation === 'horizontal';
-    const index = this._itemPositions.findIndex(({drag, clientRect}, _, array) => {
+    const index = this._itemPositions.findIndex(({drag, clientRect}) => {
+      // Skip the item itself.
       if (drag === item) {
-        // If there's only one item left in the container, it must be
-        // the dragged item itself so we use it as a reference.
-        return array.length < 2;
+        return false;
       }
 
       if (delta) {