R1:081163ab6d4f

Simplify side nav CSS and fix various glitches

Summary:
Fixes glitches in the side nav. Resolves T1828. Resolves T2156.

- Elastic scrolling (T2156): in Safari on OSX, using a scroll touch on the trackpad to scroll up past the top of the document caused newer-style side menus to scroll down, leaving a visible whitespace bar.
- Whitespace glitch: Particularly in Safari, scrolling down the document quickly from the top caused the top menu to scroll away before the side menu rose to meet it. Use a fixed…