Cheers, fellow Kastronauts!
When building rockets if I turn on Advanced Options with the filters by size and subassemblies... once I do that I can no longer select any objects, nor get back to simple option... it is sort of dead until I go to another building and then come back...
As long as I don't do the advanced everything works fine. I'm assuming it's some mod since I never used to have this problem when playing stock, but trying to uninstall and reinstall 50 mods one at a time is a pain, so if someone has had this and knows what the cause is, please let me know!
I did fix another similar problem by going back to dx9 rather than dx11. But that may be unrelated.