Fixing resizing performance issue in split view 

Resizing in split view is too slow because the width of the codemirror editor is changing on resizing and it has to render it's contents every time.

We change the width to fixed size (3000px) on resizing start and restore to default at the end so codemirror doesn't render it's contents anymore :)
This is not the Closure-style [type annotation](https://developers.google.com/closure/compiler/docs/js-for-compiler#types) that we're using. It should start with @param {boolean} makeFixed.