Fix an infinite loop if the window manager pings Emacs during DND

* src/xterm.c (handle_one_xevent): Work around _NET_WM_PING
infinite loop during drag and drop.

src/xterm.c

@@ -11014,7 +11014,19 @@ handle_one_xevent (struct x_display_info *dpyinfo,
 
 		send_event.xclient.window = dpyinfo->root_window;
 		XSendEvent (dpyinfo->display, dpyinfo->root_window, False,
-			    SubstructureRedirectMask | SubstructureNotifyMask,
+			    /* FIXME: handling window stacking changes
+			       during drag-and-drop requires Emacs to
+			       select for SubstructureNotifyMask,
+			       which in turn causes the message to be
+			       sent to Emacs itself using the event
+			       mask specified by the EWMH.  To avoid
+			       an infinite loop, just use
+			       SubstructureRedirectMask when a
+			       drag-and-drop operation is in
+			       progress.  */
+			    ((x_dnd_in_progress || x_dnd_waiting_for_finish)
+			     ? SubstructureRedirectMask
+			     : SubstructureRedirectMask | SubstructureNotifyMask),
 			    &send_event);
 
 		*finish = X_EVENT_DROP;