Use cached selector for $(window).

Mostly for consistency with the rest of the code around here.

(imported from commit 87539fea96286e1563624efd2a4800df88f2199c)

zephyr/static/js/ui.js

@@ -133,7 +133,7 @@ function resizehandler(e) {
         sidebar_nav.width('');
     }
 
-    $("#bottom_whitespace").height($(window).height() * 0.4);
+    $("#bottom_whitespace").height(viewport.height() * 0.4);
     $("#main_div").css('min-height', viewport.height());
 
     // This function might run onReady (if we're in a narrow window),