How to reproduce:
1. Open the “TreeViewBug.zip“ project
2. Open the “Editor“ Scene
3. Enter Play Mode
4. To the left of the Game view, in the TreeView Hierarchy, drag the “GameCamera“ element over the “Cube(Clone)“ element (until it expands)
5. Drop the held element between the two Hierarchy elements
6. Observe the Console

Expected result: No error is thrown
Actual result: An “InvalidOperationException“ error is thrown

Reproducible in: 2023.3.0a19, 6000.0.43f1, 6000.1.0b11, 6000.2.0a7
Could not test in: 2022.1.0a1, 2022.3.60f1, 2023.3.0a2 (Unsolvable Git package dependency issues)

Reproduced on: Windows 11 Pro (24H2), macOS 15.3.1 (M1)
Not reproduced on: No other environment tested

Notes:
- Reproducible in the Player
- This does not happen in the Editor’s Hierarchy
- Part of the error reads:
“InvalidOperationException: HierarchyFlattened was modified.
UnityEngine.UIElements.BaseTreeViewController+<GetAllItemIds>d__23.MoveNext () (at <e3bfbd1d38e94b5b8af57db71bd4720f>:0)“