fix(cdk/drag-drop): item returned to wrong index in initial container (#32944)

Fixes that when we return the item to its original container, we may end up putting it at the wrong index, because the logic that de-duplicates the item runs too late.

Fixes #32940.

Author: crisbeto

src/cdk/drag-drop/sorting/single-axis-sort-strategy.ts

@@ -168,16 +168,24 @@ export class SingleAxisSortStrategy implements DropListSortStrategy {
    *   out automatically.
    */
   enter(item: DragRef, pointerX: number, pointerY: number, index?: number): void {
+    const activeDraggables = this._activeDraggables;
+    const currentIndex = activeDraggables.indexOf(item);
+    const placeholder = item.getPlaceholderElement();
+
+    // Since the item may be in the `activeDraggables` already (e.g. if the user dragged it
+    // into another container and back again), we have to ensure that it isn't duplicated.
+    // Note that we need to run this early so the code further below isn't thrown off.
+    if (currentIndex > -1) {
+      activeDraggables.splice(currentIndex, 1);
+    }
+
     const newIndex =
       index == null || index < 0
         ? // We use the coordinates of where the item entered the drop
           // zone to figure out at which index it should be inserted.
           this._getItemIndexFromPointerPosition(item, pointerX, pointerY)
         : index;
 
-    const activeDraggables = this._activeDraggables;
-    const currentIndex = activeDraggables.indexOf(item);
-    const placeholder = item.getPlaceholderElement();
     let newPositionReference: DragRef | undefined = activeDraggables[newIndex];
 
     // If the item at the new position is the same as the item that is being dragged,
@@ -197,12 +205,6 @@ export class SingleAxisSortStrategy implements DropListSortStrategy {
       newPositionReference = activeDraggables[0];
     }
 
-    // Since the item may be in the `activeDraggables` already (e.g. if the user dragged it
-    // into another container and back again), we have to ensure that it isn't duplicated.
-    if (currentIndex > -1) {
-      activeDraggables.splice(currentIndex, 1);
-    }
-
     // Don't use items that are being dragged as a reference, because
     // their element has been moved down to the bottom of the body.
     if (newPositionReference && !this._dragDropRegistry.isDragging(newPositionReference)) {