I build Emacs from master, on Ubuntu 20.04 with i3wm. Just updated my build & am seeing the oddest problem: individual Emacs frames will freeze: no UI refresh, no response to keyboard input. Other frames are fine; emacsclient connects & works fine.
Since the program itself isn't hung or locked up my usual bag of tricks (toggle-debug-on-{error,quit}, attach gdb, &c) is no help. Nothing in *Message* or *Warnings*.
I'm now in the process of stripping down my "instance under test", getting it closer & closer to a plain/vanilla Emacs to see if the problem stops. It's made me realize I don't really know much about the Emacs UI implementation, so this question is not a request for debugging help so much as pointers on how Emacs services input & handles UI updates on a per-frame basis.