> And it's not what my previous complaints were about. A drift is not
 > simply a lag. Lag rights itself at the end of the cursor motion (you
 > just have to wait a little). Drift doesn't right itself: the cursor
 > ends up at a certain distance from the the point of the frame it has
 > been dragging.

I've rewritten the mouse code completely, patch attached.  It neither
fixes the GTK child frame resizing problem nor the jumping issues in
Lucid but should provide better dragging of frames via internal borders
and mode-/header-line otherwise.  In particular the drift you mentioned
above should be gone now.  Please try it.

martin