Portable pane <-> screen coordinate conversion

I'm trying to find a portable way within CAPI to translate from pane-to-screen 
and screen-to-pane coordinates. For instance, a non-portable way would be to 
use WIN32:SCREEN-TO-CLIENT call. 

My goal is a drag-drop facility between panes in differnt interfaces. Mouse
tracking isn't a problem. But, the "drop" coordinates are, of course, relative
to the source pane. 

Thoughts? 

Thanks, 
Greg