How to reproduce:
1. Download and extract the "1315831 repro" project into a Perforce Workspace
2. In P4V Add and Submit the Assets, Project Settings and Packages folders.
3. Open the project and set up the Perforce version control in the Project Settings (while using the same Workspace as in step 1)
4. In P4V switch the Workspace into a different one
5. Get the latest version of the project
6. Find the "Cube.prefab" file, select it and in the right-click menu select "Change Filetype"
7. Add the Exclusive Checkout option (+l) and press OK
8. Submit both files
9. While still using a different workspace, checkout the "Cube.prefab" file
10. Go back to the Editor and open the "SampleScene" Scene from Assets > Scenes
11. Open the "Cube" Prefab in the Prefab Mode
11. Make a change to the Prefab
12. Click the back arrow in the Hierarchy window to quit the Prefab Mode
13. In the pop up click "Save" -> "Check Out"
Expected results: the Editor UI is still visible
Actual results: the Editor UI turns black and multiple errors are thrown to the Console window
Reproducible with: 2020.2.0a16, 2020.2.5f1, 2021.1.0b9, 2021.2.0a7
Not reproducible with: 2018.4.32f1, 2019.4.22f1, 2020.2.0a15
Notes:
-Make sure that the "Version Control Check Out Failed" windows is closed in the 13th step
- Full error message that's printed 7 times upon reproducing the issue:
UIElement is not compatible with IMGUI GUIClips, only GUIClip.ParentClipScope
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)