Broadway docs or code is broken
The docs at https://gitlab.gnome.org/GNOME/gtk/blob/main/docs/reference/gtk/broadway.md seem to imply that a broadway server is only needed to run multiple applications, but that doesn't work:
$ unset DISPLAY
$ GDK_BACKEND=broadway gtk4-demo
(gtk4-demo:1491135): Gtk-WARNING **: 12:22:55.474: cannot open display:
Either the docs or the code needs fixing.