To reproduce:
- Create two BSP box brushes
- Extrude one box vertically and horizontally (to get an L shape)
- Select all front faces on each, starting from the L-shaped box (ctrl+left click, may require clicking twice on the second brush to first select it and then select the face)
- Click on the 3D widget to extrude
- Editor freezes, release mouse button
- Crash
Haven’t managed to find a minimal working example other than the L-shaped box, which consistently crashes my editor. It also crashes, albeit randomly, on different more complex brush configurations.