Because they get set from the Event thread. Example:
extensions [xw]
to setup
ca
xw:create-tab "t1"
show [ xw:background ] xw:of "t1"
end
Extension exception: Property BACKGROUND does not exist for widget T1.
error while observer running XW:BACKGROUND
called by procedure SETUP
called by Command Center
Because they get set from the Event thread. Example: