Negotiation of the correct action for Motif DND is broken
both for source side.

It looks like the problem is that GDK's understanding of
how this works for Motif just isn't right. What GTK+
assumes is:

 Source sends mask of possible actions to the dest
 Dest picks 

But actually the mask of possible actions that the dest sends
back is relevant in some fashion. GTK+ always sends back
a possible action mask of 0.

Looks like the best approach for debugging this would be
to protocol-snoop two Motif apps talking to each other.