The “no official support” thing is really only there because I don’t drive X11 and don’t test for bugs there myself. Anecdotally, it works about the same feature/bug-wise.
Very nice to hear some feedback! I see that the “shifting windows by moving them” thing is pretty popular, I’ll have to look into something to do that.
This is a script for KWin, not a replacement for KWin (you use the script with KWin)
https://www.reddit.com/r/kde/comments/6uv167/is_there_a_kde_equivalent_to_gnome_boxes/
Tldr - Virtualbox uses qt5 and virt manager is also supported