ui: Clean up scrollbar component (#35121)

This PR does some minor cleanup to the scrollbar component. Namely, it
removes some clones, reduces the amount of unnecessary notifies and
ensures the scrollbar hover state is more accurately updated.

Release Notes:

- N/A
e911364664Finn Evers committed on 7/26/2025, 12:08:20 AM· committed by GitHubparent4854f83
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