Ability to make side nav content automatically snap under the sub-header component instead of the main header component

**I'm submitting a ...**

* [ ] bug report
* [x] feature request
* [ ] question about the decisions made in the repository

### Issue description
I have created two side panels with fixed positions, to emulate "floating" side panels that are collapsible. The sidenav content is automatically positioned in a way to display directly under the main header component (if there is one). If I create a sub-header component that spans the width of the entire screen (just like a normal header), I would like my side nav content to be positioned underneath the _sub header_ instead of the header. Currently, the "floating panels" just sit on top of the sub header because they are snapped to position under the main header instead, defeating the purpose of a sub header.