can`t scroll left sidebar if use fixed layout

i add `fixed` class to `<body>` to use [fixed layout template](https://adminlte.io/themes/AdminLTE/pages/layout/fixed.html). But now left sidebar (nav menu) has `position: fixed` and i can't scroll it. To fix it AdminLTE use `jquery.slimscroll.min.js` bower asset library. Think if user want to use fixed layout, assets need include this js lib.