> With Emacs 25.3 under MacOS 10.12, a drag with mouse-1 depressed from
 > the text area of frame F1 to the text area of frame F2 improperly
 > generates a drag event whose (posn-window (event-end <event>)) shows
 > F1 rather than F2.
 >
 > Note that for a drag between frames, posn-window should return a frame
 > (according to the Elisp manual but not its own doc string).  The bug is
 > that the event itself records the wrong frame (the depress frame rather
 > than the release frame).
 >
 > I have confirmed this with Emacs 25.2 under Windows 7 as well.

I tried to address this problem in the attached patch.  Tested with
GTK, Lucid, Motif and Windows builds.  Since my GNUstep Emacs is
currently broken, somebody please verify that it does something
reasonable (if anything at all) on MacOS.  Otherwise, I'd need help
from people working there.

The patch should also fix the mouse drag and drop region vs. mouse
avoidance mode problem.  Please someone verify that it does TRT now.

TIA, martin