To add on the accepted answer, it's better to use "if website.user_id != user_id else" instead of "if editable". Because when it's "if editable",  if your website has more than one language installed, you won't be able to translate them, because the navbar overlaps the editor navbar. 

My navbar tag: 

<div t-att-class="'navbar navbar-static-top' if website.user_id != user_id else 'navbar navbar-fixed-top'">

Add this to body tag:

body {

min-height: 2000px;

padding-top: 70px;

}

The problem is from bootstrap, not odoo. When you fix top, the container adapts, so you need to add the padding, so it restores to the previous size.