Better handle drag-and-drop from one Emacs frame to another

* doc/lispref/frames.texi (Drag and Drop): Document new
parameter `return-frame' to `x-begin-drag'.
* lisp/mouse.el (mouse-drag-and-drop-region): Utilize new
feature.

* src/xfns.c (Fx_begin_drag): New parameter `return-frame'.
* src/xterm.c (x_dnd_begin_drag_and_drop): New parameter
return_frame_p.
(handle_one_xevent): Set new flags and return frame whenever
appropriate.
* src/xterm.h: Update prototypes.

1 files changed, 7 insertions, 1 deletions

diff --git a/doc/lispref/frames.texi b/doc/lispref/frames.texi
index 38897d6a0b3..ea5dd4c675b 100644
--- a/doc/lispref/frames.texi
+++ b/doc/lispref/frames.texi
@@ -4042,7 +4042,7 @@ you want to alter Emacs behavior, you can customize these variables.
   On some window systems, Emacs also supports dragging contents from
 itself to other frames.
 
-@defun x-begin-drag targets action &optional frame
+@defun x-begin-drag targets action &optional frame return-frame
 This function begins a drag from @var{frame}, and returns when the
 session ends, either because the drop was successful, or because the
 drop was rejected.  The drop occurs when all mouse buttons are
@@ -4061,6 +4061,12 @@ the drop target, or @code{XdndActionMove}, which means the same as
 @code{XdndActionCopy}, but also means the caller should delete
 whatever was saved into that selection afterwards.
 
+If @var{return-frame} is non-nil and the mouse moves over an Emacs
+frame after first moving out of @var{frame}, then that frame will be
+returned immediately.  This is useful when you want to treat dragging
+content from one frame to another specially, while also being able to
+drag content to other programs.
+
 If the drop was rejected or no drop target was found, this function
 returns @code{nil}.  Otherwise, it returns a symbol describing the
 action the target chose to perform, which can differ from @var{action}