The `CloseButton` component in @hypothesis/frontend-shared has styling that
makes it taller (44px) on touch devices, but the `Tab` component does not. Items
in the `TabHeader` are centered on the cross axis, so on mobile, the close
button became taller than the tabs and detached them from the tab panel.

An alternative way to fix this would be to make tabs also grow in height on
touch devices, but they are quite usable without.

Fixes https://github.com/hypothesis/client/issues/6131