Fixes #45

[nwalke]

This might not be the best way to do this, but it certainly gets the job done.

Here's a gif of it in action: https://i.imgur.com/9mgpwHC.gif

[ssddanbrown]

Thanks again for the contribution! I changed the tree-list to be more of a full-screen sidebar as there were some issues when the tree list was taller than the page. I also changed some of the JS so it took into account the mobile widths and other breaking factors.

Here's a video of it now:
https://gfycat.com/TintedLastingEuropeanpolecat

I'm not fully set on it but I'll see how it feels in daily use.

[nwalke]

Ah, perfect. I also realized my version was setting the CSS on every scroll action... probably not the best.