System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 970/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 445.87
Blender Version
Broken: version: 2.83 (sub 15), branch: master, commit date: 2020-04-25 17:40, hash: rBf418eddf8e1c
Worked: Can't find any, earliest version which I've tried was 2.74 and it behaves the same.
Short description of error
If user enable full screen mode and save this scene as a startup file, it wouldn't be possible to return Blender to the windowed mode after the start from this file. If scene was changed and then user trying to disable fullscreen, it's impossible to close Blender, since it's apparently asking if file should be saved and the window itself never appearing.
Demo: https://youtu.be/Ih56mCo_3ks
As you can see from the video, new preview of this window aren't rendering, so there's literally no window anymore.
Exact steps for others to reproduce the error
- Press Window -> Toggle Window Fullscreen
- Save startup file
- Restart Blender
- Toggle Window Fullscreen
Congrats, it's stuck and can be only closed through Task Manager.

