[PATCH weston 4/6] xdg-shell: Send an error when the client uses the not-topmost popup

[Bill Spitzak]

Are you sure this supports a client replacing the top popup with a 
different one? It seems to set the top to null rather than the next one 
down when one is removed. I would also be concerned that it may be 
difficult for a client to insure that the previous popup is destroyed 
before the new one is created, some possible toolkit designs may like to 
do all the creation first, then the destruction, to allow reuse of 
shared data.