Fixes: debbugs:18939

* simple.el (deactivate-mark): Only modify PRIMARY if we own PRIMARY.

lisp/ChangeLog

+2015-03-22  Jan Djärv  <jan.h.d@swipnet.se>
+
+	* simple.el (deactivate-mark): Only modify PRIMARY if we own
+	PRIMARY (Bug#18939).
+
 2015-03-22  Martin Rudalics  <rudalics@gmx.at>
 
 	* emacs-lisp/debug.el (debug): Don't try using "previous" window

lisp/simple.el

@@ -4420,7 +4420,8 @@ run `deactivate-mark-hook'."
       ;; the region prior to the last command modifying the buffer.
       ;; Set the selection to that, or to the current region.
       (cond (saved-region-selection
-	     (x-set-selection 'PRIMARY saved-region-selection)
+	     (if (x-selection-owner-p 'PRIMARY)
+		 (x-set-selection 'PRIMARY saved-region-selection))
 	     (setq saved-region-selection nil))
 	    ;; If another program has acquired the selection, region
 	    ;; deactivation should not clobber it (Bug#11772).