Scrolling horizontally causes UI elements to clash with TreeView in aside #93

Seen on the docs page - don't know if it's specific to that or more of a global concern.

Observed behavior:

When the page is wide enough to scroll horizontally - for instance, opening the "Show Code" on the Tabs view - some 98 UI elements draw over the TreeView located in the aside.

Expected behavior:

Either:

• Easiest: UI elements should draw underneath the TreeView in a fixed-position aside (aside z-indexes above the rest)
• Most proper: Aside should be free to scroll left and right, but remain fixed vertically.

This is also observed downstream, in XP.css - for example. A report has been made there as well, a little while ago.

Example below: