
Due to !2056 the monitor config is 2 instead of 4 on the desktop. However, the constant `Meta.MonitorSwitchConfigType.UNKNOWN` is fixed to 4, and may cause the initial selection to be out of index thus fail to display the switcher. This replaces the above constant to the actual length of the switcher items. Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2451>