bug#19988: 25.0.50; Drag events ending in different frame

[Eli Zaretskii]

> From: Stefan Monnier <monnier@IRO.UMontreal.CA>
> Cc: Tassilo Horn <tsdh@gnu.org>, 19988@debbugs.gnu.org
> Date: Tue, 03 Mar 2015 23:28:25 -0500
> 
> > If this is only about the case when the drag ends outside of any Emacs
> > frame, then we could amend the code to cover the case that the drag
> > ends on an Emacs frame.  The current code doesn't check whether we the
> > button-up event was received in an Emacs frame.
> 
> FWIW, the current behavior is probably useful in the case where the drag
> is used in order to select a region.  IOW, we should probably include
> *both* informations.

How do we include both in a backward-compatible way?