The problem here is that there is no way communicated to the rest of the content in the container below the nav that the nav bar has been fixed to the top. You can achieve this using more modern CSS, but be aware that this won't be a fix for older browsers (and indeed there are issues you may find with postion:fixed properties in older browsers too...
.affix + .container {
padding-top:50px
}
This waits until the nav bar is fixed, and then adds padding to the container that is it's sibling, keeping it from "jumping" under the nav.
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