Woop, a HACKING document for openbox at last!

Beware the Client.transient_for. It can be set to a !NULL value of TRAN_GROUP,
which is not a valid pointer. You must ALWAYS check for TRAN_GROUP before
following transient_for. When TRAN_GROUP is found, Client.group will always
be !NULL. Some smart action should be taken using all members of the group in
this case.