This required adding scrollbar support to `list`. Since `list` is
virtualized, the scrollbar height will change as more items are
measured. When the user manually drags the scrollbar, we'll persist the
initial height and offset calculations accordingly to prevent the
scrollbar from moving away from the cursor as new items are measured.
We're not doing this yet, but in the future, it'd be nice to budget some
time each frame to layout unmeasured items so that the scrollbar height
is as accurate as possible.
Release Notes:
- N/A
7b40ab30d7