making a complete Transparent Cairo context

Hi,

I am trying to make a complete transparent Cairo context while experimenting with a replacement for the standard ToolTip as you can see here.

http://screencast.com/t/w6fDRkzX5JR

In the video, I manage to make a cairocontext of appropriate size, but I don't manage to get rid of its black bounds and white background color.

It seems that this has something to do with how the context is created but these method go beyond my understanding.

Regards,

Maarten,