How to reproduce:
1. Open any project with UI Toolkit installed
2. In the Project Browser create a UI Toolkit Document and open it
3. If the UI Builder is opened in a docked window undock it
4. Now dock the window anywhere
5. Undock it and observe the .uxml background in the Viewport

Actual result: The background for the .uxml file is no longer transparent and turns white in the Viewport
Expected result: The background remains transparent

Reproducible with: 2023.2.0a18, 2023.2.0b6, 2023.3.0b2
Not reproducible with: 2021.3.29f1, 2022.3.7f1, 2023.1.9f1, 2023.2.0a17

Note: The issue might appear immediately after docking, the issue disappears if the window is closed and opened again

Reproduced on: Windows 11