Perf UX : reduce page paint on scrolling, due to fixed header element union with progress bar

Should provide some performance improvement for scrolling on chrome
The fixed header and the fixed progress bar on both ends  of the screen will invoke a complete visible page redraw.

For whatever reason this css should make things better for low performance devices. only tested on chrome superficially.

app/assets/stylesheets/common/base/header.scss

@@ -8,6 +8,7 @@
 
     .docked & {
       position: fixed;
+      backface-visibility: hidden;  /** do magic for scrolling performance **/
     }
 
     .contents {