The background color of the plugin extends to some panel widgets when run as internal

When plugin is run as internal (this is the default on wayland now, or it can be forced by setting /force-all-internal to true in xfce4-panel channel), its background color extends to certain plugins (e.g. xfce4-mpc-plugin), and certain dialog widgets, such as the tabs in the preferences dialog, or the scrollbar in the add items dialog. I thought this had been fixed with #73 (closed), but apparently not completely.

Version: 1.12.0+19+g549638a