fix(drag-drop): clear duplicate ids from descendants (#15135)

Fixes `CdkDrag` generating elements with duplicate ids, if one of its descendants has an id.

Fixes #15120.

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

@@ -1392,6 +1392,21 @@ describe('CdkDrag', () => {
       expect(preview.getAttribute('id')).toBeFalsy();
     }));
 
+    it('should clear the ids from descendants of the preview', fakeAsync(() => {
+      const fixture = createComponent(DraggableInDropZone);
+      fixture.detectChanges();
+      const item = fixture.componentInstance.dragItems.toArray()[1].element.nativeElement;
+      const extraChild = document.createElement('div');
+      extraChild.id = 'child-id';
+      extraChild.classList.add('preview-child');
+      item.appendChild(extraChild);
+
+      startDraggingViaMouse(fixture, item);
+
+      expect(document.querySelectorAll('.preview-child').length).toBeGreaterThan(1);
+      expect(document.querySelectorAll('[id="child-id"]').length).toBe(1);
+    }));
+
     it('should not create a preview if the element was not dragged far enough', fakeAsync(() => {
       const fixture = createComponent(DraggableInDropZone, [], 5);
       fixture.detectChanges();
@@ -1597,6 +1612,21 @@ describe('CdkDrag', () => {
       expect(placeholder.getAttribute('id')).toBeFalsy();
     }));
 
+    it('should clear the ids from descendants of the placeholder', fakeAsync(() => {
+      const fixture = createComponent(DraggableInDropZone);
+      fixture.detectChanges();
+      const item = fixture.componentInstance.dragItems.toArray()[1].element.nativeElement;
+      const extraChild = document.createElement('div');
+      extraChild.id = 'child-id';
+      extraChild.classList.add('placeholder-child');
+      item.appendChild(extraChild);
+
+      startDraggingViaMouse(fixture, item);
+
+      expect(document.querySelectorAll('.placeholder-child').length).toBeGreaterThan(1);
+      expect(document.querySelectorAll('[id="child-id"]').length).toBe(1);
+    }));
+
     it('should not create placeholder if the element was not dragged far enough', fakeAsync(() => {
       const fixture = createComponent(DraggableInDropZone, [], 5);
       fixture.detectChanges();

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

@@ -1003,8 +1003,15 @@ function getTransform(x: number, y: number): string {
 /** Creates a deep clone of an element. */
 function deepCloneNode(node: HTMLElement): HTMLElement {
   const clone = node.cloneNode(true) as HTMLElement;
+  const descendantsWithId = clone.querySelectorAll('[id]');
+
   // Remove the `id` to avoid having multiple elements with the same id on the page.
   clone.removeAttribute('id');
+
+  for (let i = 0; i < descendantsWithId.length; i++) {
+    descendantsWithId[i].removeAttribute('id');
+  }
+
   return clone;
 }