Reproduction steps:
1. Open users attached project in "Oculus-20-Lights-Base.zip"
2. Completely close and re-open ProBuilder Window
3. Adjust ProBuilder Window Size by maximizing, reducing or adjusting the edges of the Window
4. Change ProBuilder Window type from dockable/floating (sometimes docking/undocking the window helps)
5. Switch ProBuilder Selection Mode in Scene (Object/Vertex/Edge/Face) (check video)
6. Adjust ProBuilder Window Size by maximizing, reducing or adjusting the edges of the Window
7. Repeat steps 3-6 (Usually < 10 minutes)

Unexpected behaviour: when adjusting ProBuilder Window size, the Window becomes Red coloured (check note)

Reproducible with: ProBuilder preview.8 - 4.3.0 (2020.1.0b5, 2020.1.0b8)
Reproducible with OpenGL and Metal

Notes:
1. The window remains functional, however, it is rendered completely red. The user mentioned that the window would sometime freeze but I was not able to reproduce the freeze
2. I was not able to test this on more versions. However, I am not sure if I was not able to reproduce it or the bug was not reproducible.