It is not a part of the foreign-toplevel-management protocol to get the
class of a toplevel, only for getting the app_id.
For xwayland clients this is an issue because that means that you cannot
identify what application the toplevel refers to which is the point of
the app_id property.
By falling back to class when an app_id does not exist solves this problem.
Phoc also uses app_id and class interchangeably in their implementation
of foreign-toplevel-management, in fact they always do that and not only
for just this protocol.
c8d8a4c544/src/xwayland.c (L236)
1f8dbb320a