To troubleshoot screen sharing, see xdg-desktop-portal-wlr FAQ

To test desktop notifications, use notify-send "foo" at the command line.

Change GUI font size (for GTK programs): gsettings set org.gnome.desktop.interface font-name 'Noto Sans, 14' Dunno yet if this is permanent. You can also use GTK font manager: sudo apt install font-manager

To run a graphical program (in this example, lshw-gtk) as root: sudo /bin/env WAYLAND_DISPLAY="$XDG_RUNTIME_DIR/$WAYLAND_DISPLAY" XDG_RUNTIME_DIR=/user/run/0 /usr/bin/lshw-gtk