Comment # 2 on bug 105651 from Alex Deucher

This is handled at the Desktop level.  There are quite a few large format
displays that show up as multiple DP MST displays, even a number of early 4k
displays.  X exposes the tile info via randr 1.5 and the desktop environment
should handle it as a single monitor.  Mutter handles this in GNOME for
example.

https://cgit.freedesktop.org/xorg/xserver/commit/?id=7e1f86d42b54fb7f6492875e47a718eaeca3069b
https://lists.x.org/archives/xorg-announce/2015-May/002605.html
https://mail.gnome.org/archives/desktop-devel-list/2015-November/msg00018.html