Disable middle click selection emulation during XDS

* doc/lispref/frames.texi (Drag and Drop): Document new
variable.
* lisp/x-dnd.el (x-dnd-do-direct-save): Disable drop emulation
during XDS drag-and-drop.

* src/keyboard.c (kbd_buffer_get_event): Handle recorded value.
* src/xterm.c (x_dnd_do_unsupported_drop): Return if new
variable is nil.
(x_dnd_send_unsupported_drop): Record value of new variable in
events.
(x_dnd_begin_drag_and_drop): Handle recorded value.
(syms_of_xterm): New variable `x-dnd-use-unsupported-drop'.

doc/lispref/frames.texi

@@ -4280,6 +4280,18 @@ will only be used if @code{"FILE_NAME"} is one of the targets given to
 to drop all supported content.
 @end defvar
 
+@defvar x-dnd-use-unsupported-drop
+When one of the @code{"STRING"}, @code{"UTF8_STRING"},
+@code{"COMPOUND_TEXT"} or @code{"TEXT"} targets is present in the list
+given to @code{x-begin-drag}, Emacs will try to use synthesized mouse
+events and the primary selection to insert the text if the drop target
+doesn't support any drag-and-drop protocol at all.
+
+A side effect is that Emacs will become the owner of the primary
+selection upon such a drop.  If that is not desired, then the drop
+emulation can be disabled by setting this variable to @code{nil}.
+@end defvar
+
 @node Color Names
 @section Color Names
 

lisp/x-dnd.el

@@ -1144,6 +1144,7 @@ ACTION is the action given to `x-begin-drag'."
   "Whether or not the drop target made a request for `XdndDirectSave0'.")
 
 (defvar x-dnd-disable-motif-protocol)
+(defvar x-dnd-use-unsupported-drop)
 
 (defun x-dnd-handle-direct-save (_selection _type _value)
   "Handle a selection request for `XdndDirectSave'."
@@ -1204,6 +1205,7 @@ was taken, or the direct save failed."
         ;; possibly work with Motif or OffiX programs.
         (x-dnd-disable-motif-protocol t)
         (x-dnd-use-offix-drop nil)
+        (x-dnd-use-unsupported-drop nil)
         (prop-deleted nil)
         encoded-name)
     (unwind-protect

src/keyboard.c

@@ -4068,7 +4068,10 @@ kbd_buffer_get_event (KBOARD **kbp,
 
 	  /* `x-dnd-unsupported-drop-function' could have deleted the
 	     event frame.  */
-	  if (!FRAME_LIVE_P (f))
+	  if (!FRAME_LIVE_P (f)
+	      /* This means `x-dnd-use-unsupported-drop' was nil when the
+		 event was generated.  */
+	      || NILP (XCAR (XCDR (XCDR (XCDR (event->ie.arg))))))
 	    break;
 
 	  x_dnd_do_unsupported_drop (FRAME_DISPLAY_INFO (f),

src/xterm.c

@@ -3810,6 +3810,9 @@ x_dnd_do_unsupported_drop (struct x_display_info *dpyinfo,
   if (NILP (value))
     return;
 
+  if (!x_dnd_use_unsupported_drop)
+    return;
+
   event.xbutton.serial = 0;
   event.xbutton.send_event = True;
   event.xbutton.display = dpyinfo->display;
@@ -3914,9 +3917,10 @@ x_dnd_send_unsupported_drop (struct x_display_info *dpyinfo, Window target_windo
   ie.kind = UNSUPPORTED_DROP_EVENT;
   ie.code = (unsigned) target_window;
   ie.modifiers = x_dnd_unsupported_event_level;
-  ie.arg = list3 (assq_no_quit (QXdndSelection,
+  ie.arg = list4 (assq_no_quit (QXdndSelection,
 				dpyinfo->terminal->Vselection_alist),
-		  targets, arg);
+		  targets, arg, (x_dnd_use_unsupported_drop
+				 ? Qt : Qnil));
   ie.timestamp = before;
 
   XSETINT (ie.x, root_x);
@@ -11377,7 +11381,10 @@ x_dnd_begin_drag_and_drop (struct frame *f, Time time, Atom xaction,
 
       /* `x-dnd-unsupported-drop-function' could have deleted the
 	 event frame.  */
-      if (!FRAME_LIVE_P (event_frame))
+      if (!FRAME_LIVE_P (event_frame)
+	  /* This means `x-dnd-use-unsupported-drop' was nil when the
+	     event was generated.  */
+	  || NILP (XCAR (XCDR (XCDR (XCDR (event->ie.arg))))))
 	continue;
 
       x_dnd_do_unsupported_drop (FRAME_DISPLAY_INFO (event_frame),
@@ -28075,4 +28082,11 @@ drag-and-drop code.  */);
 When non-nil, `x-begin-drag' will not drop onto any window that only
 supports the Motif drag-and-drop protocols.  */);
   x_dnd_disable_motif_protocol = false;
+
+  DEFVAR_BOOL ("x-dnd-use-unsupported-drop", x_dnd_use_unsupported_drop,
+    doc: /* Enable the emulation of drag-and-drop based on the primary selection.
+When nil, do not use the primary selection and synthetic mouse clicks
+to emulate the drag-and-drop of `STRING', `UTF8_STRING',
+`COMPOUND_TEXT' or `TEXT'.  */);
+  x_dnd_use_unsupported_drop = true;
 }