fix(cdk/drag-drop): incorrect index when returning item in mixed list (#31592)

When an item enters a mixed orientation list, we were figuring out its index, removing the item from the list and then trying to insert it in that index. The problem is that by removing the item, we might have changed the array in a way where the index isn't valid anymore.

These changes resolve the issue by removing the item first before making any index calculations.

Fixes #31505.

Author: crisbeto

src/cdk/drag-drop/sorting/mixed-sort-strategy.ts

@@ -141,6 +141,14 @@ export class MixedSortStrategy implements DropListSortStrategy {
    *   out automatically.
    */
   enter(item: DragRef, pointerX: number, pointerY: number, index?: number): void {
+    // Remove the item from current set of items first so that it doesn't throw off the indexes
+    // further down in this method. See https://github.com/angular/components/issues/31505
+    const currentIndex = this._activeItems.indexOf(item);
+
+    if (currentIndex > -1) {
+      this._activeItems.splice(currentIndex, 1);
+    }
+
     let enterIndex =
       index == null || index < 0
         ? this._getItemIndexFromPointerPosition(item, pointerX, pointerY)
@@ -154,11 +162,6 @@ export class MixedSortStrategy implements DropListSortStrategy {
     }
 
     const targetItem = this._activeItems[enterIndex] as DragRef | undefined;
-    const currentIndex = this._activeItems.indexOf(item);
-
-    if (currentIndex > -1) {
-      this._activeItems.splice(currentIndex, 1);
-    }
 
     if (targetItem && !this._dragDropRegistry.isDragging(targetItem)) {
       this._activeItems.splice(enterIndex, 0, item);