Scroll bar needed for navigation frame

[ksbhaskar]

If the navigation frame exceeds the height of the window, it should have a scroll bar.

[communikit]

Found this open issue in the sphinx_rtd_theme Github repo:

readthedocs/sphinx_rtd_theme#468

where the scrollbar is intentionally hidden in the theme, and it hasn't been enabled to show. The sidebar as is, is still scrollable if the navigation frame exceeds the height of the window. There just isn't a displayed scroll bar.