I have tried unchecking Edit->Editor Preferences->Level Editor(Miscellaneous)->Update BSP Automatically which resulted in getting occasional crashes when working on BSP after multiple level rebuilds. If I enable automatic BSP build again I am not getting any BSP related crashes anymore.
Especially related to stairs since this is where I got all crashes.
This happens in 4.4 (might have happened in earlier version too as I did not test it there).
EDIT:
How to reproduce it:
- Open new level
- Uncheck Edit->Editor Preferences->Level Editor(Miscellaneous)->Update BSP Automatically
- Drag BSP linear stair into level
- Switch to Geometry Editing (Shift+5)
- Switch to rotation gizmo (press Space)
- Rotate stair (for example 90°)
- Change Num Steps to something like 3 in the Details window
- Try to switch back to translation gizmo (press Space)
- Editor crashes upon right-clicking into the viewport and moving mouse a little so that it gets focus so that you can switch gizmo