On pages without full-page content mode, the page wasn't properly set up

to allow the mobile sidebar to scroll. The sidebar's height was bound to

the #container height, which would be based on the content height.

Only in full-page content mode would the #container height be bound to

the viewport.
This adds a couple of extra rules to activate the correct container

height when the mobile menu is open, and to always enable sidebar

scrolling in mobile mode, ensuring that the UI always behaves as

expected regardless of the modes used on the page.

Tested the mobile scrollbar behavior on mobile mode in pages with both

standard and full-page content modes.