tabwin: Comply with the primary monitor setting
If the user selected a primary monitor, the Alt+Tab window will only appear on that monitor, otherwise on all of them.
To quote the GDK documentation for gdk_display_get_primary_monitor:
[...], specialized desktop applications such as panels should place themselves on the primary monitor.
It is however not possible to use gdk_display_get_primary_monitor and also respect the user's setting. Despite what the online documentation says, recent versions of GTK+3 may no longer return a null value and will instead pick one monitor as the primary if there isn't already one defined. The nullable marker  for the return value was eventually dropped.
Until Wayland support becomes a requirement or until GTK+3's minimum acceptable version increases to the point where the new behavior is guaranteed, this can be conditionally solved for X11 via the RandR extension.