Disable unrelated drag-and-drop protocols during XDS drop

* doc/lispref/frames.texi (Drag and Drop): Document variables
used to control drag-and-drop protocols.
* lisp/x-dnd.el (x-dnd-do-direct-save): Disable irrelevant DND
protocols.

* src/xterm.c (x_dnd_get_target_window_1):
(x_dnd_get_target_window):
(handle_one_xevent): Respect new variable.
(syms_of_xterm): New variable `x-dnd-disable-motif-protocol'.

doc/lispref/frames.texi

@@ -4258,6 +4258,28 @@ chosen by the target.  For example, callers should delete the buffer
 text that was dragged if this function returns @code{XdndActionMove}.
 @end defun
 
+@cindex drag and drop protocols, X
+
+  On X Windows, several different drag-and-drop protocols are
+supported by @code{x-begin-drag}.  When dragging content that is known
+to not be supported by a specific drag-and-drop protocol, it might be
+desirable to turn that protocol off, by changing the values of the
+following variables:
+
+@defvar x-dnd-disable-motif-protocol
+When this is non-@code{nil}, the Motif drag and drop protocols are
+disabled, and dropping onto programs that only understand them will
+not work.
+@end defvar
+
+@defvar x-dnd-use-offix-drop
+When this is @code{nil}, the OffiX (old KDE) drag and drop protocol is
+disabled.  When this is the symbol @code{files}, the OffiX protocol
+will only be used if @code{"FILE_NAME"} is one of the targets given to
+@code{x-begin-drag}.  Any other value means to use the OffiX protocol
+to drop all supported content.
+@end defvar
+
 @node Color Names
 @section Color Names
 

lisp/x-dnd.el

@@ -1143,6 +1143,8 @@ ACTION is the action given to `x-begin-drag'."
 (defvar x-dnd-xds-performed nil
   "Whether or not the drop target made a request for `XdndDirectSave0'.")
 
+(defvar x-dnd-disable-motif-protocol)
+
 (defun x-dnd-handle-direct-save (_selection _type _value)
   "Handle a selection request for `XdndDirectSave'."
   (setq x-dnd-xds-performed t)
@@ -1198,6 +1200,10 @@ was taken, or the direct save failed."
         (x-dnd-xds-current-file nil)
         (x-dnd-xds-source-frame frame)
         (x-dnd-xds-performed nil)
+        ;; The XDS protocol is built on top of XDND, and cannot
+        ;; possibly work with Motif or OffiX programs.
+        (x-dnd-disable-motif-protocol t)
+        (x-dnd-use-offix-drop nil)
         (prop-deleted nil)
         encoded-name)
     (unwind-protect

src/xterm.c

@@ -3542,6 +3542,7 @@ x_dnd_get_target_window_1 (struct x_display_info *dpyinfo,
      root_x and root_y are.  */
 
   *motif_out = XM_DRAG_STYLE_NONE;
+
   for (tem = x_dnd_toplevels; tem; tem = tem->next)
     {
       if (!tem->mapped_p || tem->wm_state != NormalState)
@@ -3616,7 +3617,9 @@ x_dnd_get_target_window_1 (struct x_display_info *dpyinfo,
 
   if (chosen)
     {
-      *motif_out = chosen->xm_protocol_style;
+      *motif_out = (x_dnd_disable_motif_protocol
+		    ? XM_DRAG_STYLE_NONE
+		    : chosen->xm_protocol_style);
       return chosen->window;
     }
   else
@@ -4147,7 +4150,8 @@ x_dnd_get_target_window (struct x_display_info *dpyinfo,
 	      || proto != -1 || motif != XM_DRAG_STYLE_NONE)
 	    {
 	      *proto_out = proto;
-	      *motif_out = motif;
+	      *motif_out = (x_dnd_disable_motif_protocol
+			    ? XM_DRAG_STYLE_NONE : motif);
 	      *toplevel_out = child_return;
 	      x_uncatch_errors ();
 
@@ -18925,6 +18929,7 @@ handle_one_xevent (struct x_display_info *dpyinfo,
 
 			if (!xm_read_drag_receiver_info (dpyinfo, x_dnd_last_seen_window,
 							 &drag_receiver_info)
+			    && !x_dnd_disable_motif_protocol
 			    && drag_receiver_info.protocol_style != XM_DRAG_STYLE_NONE
 			    && (x_dnd_allow_current_frame
 				|| x_dnd_last_seen_window != FRAME_OUTER_WINDOW (x_dnd_frame)))
@@ -20330,6 +20335,7 @@ handle_one_xevent (struct x_display_info *dpyinfo,
 
 			      if (!xm_read_drag_receiver_info (dpyinfo, x_dnd_last_seen_window,
 							       &drag_receiver_info)
+				  && !x_dnd_disable_motif_protocol
 				  && drag_receiver_info.protocol_style != XM_DRAG_STYLE_NONE
 				  && (x_dnd_allow_current_frame
 				      || x_dnd_last_seen_window != FRAME_OUTER_WINDOW (x_dnd_frame)))
@@ -28063,4 +28069,10 @@ This lets you inspect the contents of `XdndSelection' after a
 drag-and-drop operation, which is useful when writing tests for
 drag-and-drop code.  */);
   x_dnd_preserve_selection_data = false;
+
+  DEFVAR_BOOL ("x-dnd-disable-motif-protocol", x_dnd_disable_motif_protocol,
+    doc: /* Disable the Motif drag-and-drop protocols.
+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;
 }