Repro:
- Start Rhino 7.6.21127.19001, 2021-05-07
- File > New > No Template, just to be sure
- Go to the Materials panel (here, it is docked on the right)
- Create a Custom material, type something in the Notes, hit TAB, no problem
- Create a Physically Based material, type something in the Notes, hit TAB, Rhino hangs