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.

3 files changed, 59 insertions, 5 deletions

diff --git a/src/xfns.c b/src/xfns.c
index 0d197c1dd7d..b5d0b2c54e8 100644
--- a/src/xfns.c
+++ b/src/xfns.c
@@ -6582,7 +6582,7 @@ The coordinates X and Y are interpreted in pixels relative to a position
   return Qnil;
 }
 
-DEFUN ("x-begin-drag", Fx_begin_drag, Sx_begin_drag, 1, 3, 0,
+DEFUN ("x-begin-drag", Fx_begin_drag, Sx_begin_drag, 1, 4, 0,
        doc: /* Begin dragging contents on FRAME, with targets TARGETS.
 TARGETS is a list of strings, which defines the X selection targets
 that will be available to the drop target.  Block until the mouse
@@ -6607,9 +6607,14 @@ Emacs.  For that reason, they are not mentioned here.  Consult
 "Drag-and-Drop Protocol for the X Window System" for more details:
 https://freedesktop.org/wiki/Specifications/XDND/.
 
+If RETURN-FRAME is non-nil, this function will return the frame if the
+mouse pointer moves onto an Emacs frame, after first moving out of
+FRAME.
+
 If ACTION is not specified or nil, `XdndActionCopy' is used
 instead.  */)
-  (Lisp_Object targets, Lisp_Object action, Lisp_Object frame)
+  (Lisp_Object targets, Lisp_Object action, Lisp_Object frame,
+   Lisp_Object return_frame)
 {
   struct frame *f = decode_window_system_frame (frame);
   int ntargets = 0;
@@ -6655,7 +6660,7 @@ instead.  */)
 
   x_set_dnd_targets (target_atoms, ntargets);
   lval = x_dnd_begin_drag_and_drop (f, FRAME_DISPLAY_INFO (f)->last_user_time,
-                                    xaction);
+                                    xaction, !NILP (return_frame));
 
   return lval;
 }
diff --git a/src/xterm.c b/src/xterm.c
index 8a4344f2a4f..a3d20a9d226 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -771,6 +771,15 @@ static void x_scroll_bar_end_update (struct x_display_info *, struct scroll_bar
 #endif
 
 static bool x_dnd_in_progress;
+
+/* Whether or not to return a frame from `x_dnd_begin_drag_and_drop'.
+
+   0 means to do nothing.  1 means to wait for the mouse to first exit
+   `x_dnd_frame'.  2 means to wait for the mouse to move onto a frame,
+   and 3 means to `x_dnd_return_frame_object'.  */
+static int x_dnd_return_frame;
+static struct frame *x_dnd_return_frame_object;
+
 static Window x_dnd_last_seen_window;
 static int x_dnd_last_protocol_version;
 static Time x_dnd_selection_timestamp;
@@ -1025,7 +1034,8 @@ x_set_dnd_targets (Atom *targets, int ntargets)
 }
 
 Lisp_Object
-x_dnd_begin_drag_and_drop (struct frame *f, Time time, Atom xaction)
+x_dnd_begin_drag_and_drop (struct frame *f, Time time, Atom xaction,
+                           bool return_frame_p)
 {
   XEvent next_event;
   struct input_event hold_quit;
@@ -1054,6 +1064,10 @@ x_dnd_begin_drag_and_drop (struct frame *f, Time time, Atom xaction)
   x_dnd_mouse_rect_target = None;
   x_dnd_action = None;
   x_dnd_wanted_action = xaction;
+  x_dnd_return_frame = 0;
+
+  if (return_frame_p)
+    x_dnd_return_frame = 1;
 
   while (x_dnd_in_progress)
     {
@@ -1085,6 +1099,14 @@ x_dnd_begin_drag_and_drop (struct frame *f, Time time, Atom xaction)
         }
     }
 
+  if (x_dnd_return_frame == 3)
+    {
+      x_dnd_return_frame_object->mouse_moved = true;
+
+      XSETFRAME (action, x_dnd_return_frame_object);
+      return action;
+    }
+
   FRAME_DISPLAY_INFO (f)->grabbed = 0;
 
   if (x_dnd_wanted_action != None)
@@ -11606,6 +11628,19 @@ handle_one_xevent (struct x_display_info *dpyinfo,
                     && x_dnd_last_seen_window != FRAME_X_WINDOW (x_dnd_frame))
                   x_dnd_send_leave (x_dnd_frame, x_dnd_last_seen_window);
 
+                if (x_dnd_last_seen_window == FRAME_X_WINDOW (x_dnd_frame)
+                    && x_dnd_return_frame == 1)
+                  x_dnd_return_frame = 2;
+
+                if (x_dnd_return_frame == 2
+                    && x_window_to_frame (dpyinfo, target))
+                  {
+                    x_dnd_in_progress = false;
+                    x_dnd_return_frame_object
+                      = x_window_to_frame (dpyinfo, target);
+                    x_dnd_return_frame = 3;
+                  }
+
                 x_dnd_wanted_action = None;
                 x_dnd_last_seen_window = target;
                 x_dnd_last_protocol_version
@@ -12825,6 +12860,19 @@ handle_one_xevent (struct x_display_info *dpyinfo,
                           && x_dnd_last_seen_window != FRAME_X_WINDOW (x_dnd_frame))
                         x_dnd_send_leave (x_dnd_frame, x_dnd_last_seen_window);
 
+                      if (x_dnd_last_seen_window == FRAME_X_WINDOW (x_dnd_frame)
+                          && x_dnd_return_frame == 1)
+                        x_dnd_return_frame = 2;
+
+                      if (x_dnd_return_frame == 2
+                          && x_window_to_frame (dpyinfo, target))
+                        {
+                          x_dnd_in_progress = false;
+                          x_dnd_return_frame_object
+                            = x_window_to_frame (dpyinfo, target);
+                          x_dnd_return_frame = 3;
+                        }
+
                       x_dnd_last_seen_window = target;
                       x_dnd_last_protocol_version
                         = x_dnd_get_window_proto (dpyinfo, target);
diff --git a/src/xterm.h b/src/xterm.h
index 225aaf4cad5..9665e92a9fb 100644
--- a/src/xterm.h
+++ b/src/xterm.h
@@ -1367,7 +1367,8 @@ extern void x_scroll_bar_configure (GdkEvent *);
 
 extern void x_display_set_last_user_time (struct x_display_info *, Time);
 
-extern Lisp_Object x_dnd_begin_drag_and_drop (struct frame *, Time, Atom);
+extern Lisp_Object x_dnd_begin_drag_and_drop (struct frame *, Time, Atom,
+                                              bool);
 extern void x_set_dnd_targets (Atom *, int);
 
 INLINE int