This new hint allows compositors to know what portions of a window
will be obscured, as a region above them is opaque. For an RGB window,
possible to glean this information from the bounding shape region of
a client window, but not for an ARGB32 window. This new hint allows
clients that use ARGB32 windows to say which part of the window is
opaque, allowing this sort of optimization.

https://bugzilla.gnome.org/show_bug.cgi?id=679901