On #4069, we introduced a small improvement when hovering a left-pointed
bucket: focus the corresponding annotation card, in addition to the
anchor's highlight.

In this PR, we introduce the same improvement to the up and down-pointed
buckets.

In addition, I have realised we handled `onBlur` but not `onFocus`
events. I have added more complete support for keyboard navigation.

I substituted `onMouseMove` for `onMouseEnter` because it is triggered
less frequently.