nanogui: Thread: GrClearWindow() not working for a transparent window?

I am having a problem with clearing a window that has the trasparency 
property set.

I create a large parent window and set a pixmap as its background.
I then create child windows on it with the transparency property set.
On the children I draw gif images that also have transparent areas, and 
all is well. The images are in the children and the parents pixmap 
background is showing through like it should.
Then I call GrClearWindow(child, 0) and draw a new image in the child. 
The problem is the old image was not cleared, so now the new image is 
overlaying the old image.

Am I doing something wrong???

tj