What is the bug or the crash?

Follow up #59505 (@DelazJ)

Using a QGIS master on top of 2027519a153, there are some inconsistencies in the drag and drop behavior that prevent users from reordering subdiagrams. For instance:

• Most of the times, the dropped subdiagram will not stay in the desired position, but rather be appended (i.e., place in the last position of the list):

  dragndrop_stackeddiagrams_wrong_moves.mp4

• It's possible to obtain unwanted copies of dragged & dropped items in a reproducible way:

  dragndrop_stackeddiagrams_item_copied.mp4

Steps to reproduce the issue

1. Add vector data.
2. Go to Layer Styles --> Diagrams and choose Stacked Diagram.
3. Create a couple of sub diagrams.
4. Attempt to reorder them.

Versions

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

Tested on Ubuntu 24.04 LTS and Windows 11.