Outline vertical spacing (margins) and scrollbar visibility.

Fixed vertical margins by creating a third layer.  `outline` now contains the outline and toggle button, `outlineContent` contains just the outline content and `outlineScrollable` contains just the scrolling component (as opposed to the container with margins). Because the toggle button needs to turn only the margin container off, I had to separate).

Also, fixed vertical scrollbar hiding.

Fixes #1340 
Review @dinhviethoa 
Cc @devoncarew