🚀 go-pugleaf

I wonder if anyone out there can help me .
I have written a program in M68000 that allows me to open and manipulate a 
window . Not very hard I must admit but there was one thing I found strange.
When I moved the window , the background where the window originally was 
changes to black instead of the usual grey . Then when I move the window up
to the top right , the whole screen goes black . Next , I move the window
down to the bottom left and the screen reverts to its normal status .
Sizing the window is fine in the Y direction but not in the X direction .
Its all very strange.
I would appreciate any light that could be cast on this .
Thanks in advance.....(?)
--Andie Ness