May have stumbled upon a pesky bug;
Clicked the combo box under Advanced->Manual SteamVR Optimization for Pimax. No menu items were available, see screenshot.
I expected that clicking the downwards arrow on the combo box would exit it, but in this case nothing I did would let me out of that empty list. The UI was just stuck.
Tried clicking outside the box, hitting the physical keyboard Enter key, Tab key and multiple other VR interface buttons. Also tried the VR mouse.
PE was still running, everything was still tracking and I could recenter the unresponsive interface.
I tried combo boxes on other menus. They also only “let me out” when clicking on of the selection items on them, and not by any other means.
So, beware of empty combo boxes for they are really quicksand traps scary face.
Hopefully this isn’t a re-post of a known bug.