toc2: Improve "navigate" menu zero state

[calbach]

This currently just looks broken/bugged. Some informative text or default options (go to top/bottom) would be an improvement.

[zertrin]

Also have this on my side, but not only in "zero state", it is like that all the time.

If I drag the corner of this small weird looking menu (the thing just over the top left corner of the keyboard button in the image above) then I am able to see some content (after making the box bigger).

However, the menu bar resizes with it:

This is weird, and from a user point of view, broken.

(using Chrome or Firefox is the same)