According to the documentation, gdk_monitor_get_geometry() reports the
monitor geometry in ”application pixels”, not in ”device pixels”,
meaning that the actual device resolution needs to be scaled down by the
scale factor of the output.
x11 backend does that downscaling, whereas Wayland backend did not,
causing a discrepancy depending on the backend used.
https://bugzilla.gnome.org/show_bug.cgi?id=783995
8ef20c0e2f