The problem of the double window I don't understand well the reason why it happens, but it is solved by editing the direct access to the application.
This command must be included:
I will show you an example of how i edit PlayOnLinux application:
use flatpak and run the bellow command, it will allow flatpak apps to use your theme, no idea why canonical doesn't expose the theme and icon folders already, snaps are doa just because of the theming, then again they are also perform much slower than any other option out there
flatpak override --filesystem=~/.themes
Just had the same issue on elementary OS Hera. I've fixed it by adding this line to my .bashrc: