Tried it in EXWM, noticed a few issues

Hey @minad! Here's a quick video capture of me trying mini-popup in EXWM. There are three things I noticed:

• I use C-j and C-k as vertico-next/vertico-previous bindings in minibuffers but for some reason C-k doesn't work in the popup window (I've noticed this with Corfu too, just hadn't reported it yet)
• When I open a minibuffer popup and then switch EXWM workspaces a few times, the minibuffer will switch back to the original location in the frames on other workspaces. Ideally the minibuffer would stay connected to the original workspace it was opened on and not appear on other workspaces
• Sometimes when I run a command like describe-variable, no completion results will be displayed with an empty input. Wasn't able to capture that one on this recording, though.

Thanks for giving this a shot!

https://user-images.githubusercontent.com/79405/119261195-2de89e80-bb8b-11eb-8fbd-3ed074b96b9a.mp4

Reproduce the three reported symptoms in mini-popup under EXWM, starting with the C-j/C-k minibuffer bindings and then workspace switching. Also try commands such as describe-variable with empty input and compare the behavior with Vertico and Corfu. Done means popup keybindings work, the minibuffer remains tied to its original workspace, and completion results appear for the reported command.