Track selector scrollbar appears over check marks

[fatadel]

While working on #5798, I've noticed this issue - the scrollbar appears over the check marks inside the track selector.

So when the scrollbar is not there, it looks like this

Relevant docs: https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/scrollbar-gutter

┆Issue is synchronized with this Jira Task

[canova]

Actually scrollbar-gutter property will have no effect here, because macOS is showing the overlay scrollbar here (when user doesn't have a mouse) and not the classic one.

We should probably fix it in a similar way to how we fixed it in the timeline scrollbar.