Version as: major improvements to the scrollbar

1. Taps near the top of the editor sometimes failed to move the cursor. It turns out this was only happening soon after a drag of the scrollbar.
2. Lua Carousel's scrollbar has always felt a little jumpy. If you just tapped on the scrollbar it would move on you. Now it should feel perfectly stable until you drag.

Ironically, I was aware of issue 2, but deemphasized it as "just a usability issue". But if I'd fixed 2 promptly 1 would have been much more obvious, and have gotten diagnosed and fixed sooner. (I'd wrongly diagnosed it as a mobile-only issue.)