Why do keyboard shortcuts stop working in Ubuntu 18.04 with Gnome

Solution 1:

I don't really have great insights into this problem, but rather than leaving it without any answers, I'll share what I have. I think the comments on the question are great. Given that the problem happens after some time elapses running IntelliJ, I would suspect that some other application starts running and grabs those shortcut keys. It might even be some sort of plug-in in IntelliJ.

You might investigate any recently started processes when you first notice the problem start happening, via

ps e -o pid,start_time,command --sort start_time

If it is a transient process, that could be harder.