Improve L0 secondary navbar slide-in/out behavior on up scroll

[broadwell]

The code for the slide-in/out navbar is in src/assets/menu-scroll.js.

The offending behavior is that the secondary navbar menu, after having slid out at the beginning of an upward scroll (as desired), will unexpectedly and temporarily re-stow itself mid-scroll, especially on longer pages. Two likely triggers for this:

• Due to noisy signals/oversensitive hardware/etc., the continuation of a scroll-up swipe (via trackpad or mouse wheel) actually registers as a slight scroll-down signal followed by the continued scroll-up. The scroll-down causes the menu to hide briefly. Can we "de-noise" this behavior without adversely suppressing the desired menu slide-in for legitimate scroll-down events?
• If the user scrolls up especially quickly, it may trigger the arbitrary 100-pixel threshold that we interpret as indicating that the change in the window coordinates was caused by clicking an in-page anchor link, in which case we want the navbar to be stowed away so that it doesn't obscure the top of the anchor target. Probably a more sustainable fix would involve coordinating the menu-scroll.js code with the logic in src/assets/tabs.js?

[broadwell]

Update: pretty sure the "noisy hardware" explanation is a red herring, although the fix in #575 does include code to ignore any scroll events of less than 1 pixel. The other, more self-evident causes of navbar misbehavior should be replicable on any machine. Here are instructions for producing them on /music/otsuzumi-kotsuzumi/:

1. Scroll to the bottom of the page, then scroll up quickly and continuously. If you do it at a medium-fast pace, the sticky navbar should hide and unhide itself periodically during the scroll. Try gradually accelerating your up-scroll speed to trigger it.
2. Click on the Strict and Flexible sidebar link, then click Rhythmic Organization. This will cause an upward jump-scroll of less than 100 pixels (on most devices), at which point the navbar will unhide, obscuring the top of the content you just jumped to. The behavior is more annoying with tab anchors, but this is an easy way to replicate it.