Cache the window selector as 'viewport' in an attempt to improve performance.

(imported from commit 3e01382260938737fbee663f6a9e94ad495ef21e)

zephyr/static/js/hotkey.js

@@ -46,7 +46,6 @@ function process_hotkey(code) {
             //
             // FIXME: this doesn't work for End because get_last_visible()
             // always returns a message.
-            var viewport = $(window);
             viewport.scrollTop($("#main_div").outerHeight(true));
         }
         return process_hotkey;