Sometimes windows ignore mouse input

I need some help troubleshooting this because I don’t even know where to start looking.

Sometimes windows just ignore mouse input entirely, but are still reacting to keyboard input. One place where this happens consistently is in Thunderbird, with the app chooser dialogue when opening a link.

I’ve also seen this happen with the entire Waterfox browser window a couple of times. I don’t know how to reproduce it there though. I can’t say for sure if I’ve seen it elsewhere or not. I’m not great at remembering details.

It’s not the app hanging since it still reacts to the keyboard.

Could of course be a weird coincidence that it happens in two different apps at the same time. I do regular system upgrades (about every week).

Any ideas?