Deactivate sidebar panels when the panel is not selected.

Special thanks to matt@netscape.com for providing this patch.
Bug #53239 r=cls sr=ben a=asa

xpfe/components/sidebar/resources/sidebarOverlay.js

@@ -310,6 +310,7 @@ function (force_reload)
         header.removeAttribute('selected');
         content.setAttribute('collapsed','true');
 
+        iframe.setAttribute('src', 'about:blank');
         load_state = content.getAttribute('loadstate');
         if (load_state == 'loading') {
           iframe.removeEventListener("load", panel_loader, true);