Originally this was an intentional choice (see
https://github.com/ppy/osu/pull/18088) when these controls were more
transparent and didn't for a solid toolbox area.
But this is no longer the case, so for now let's always block scroll to
match user expectations.
Closes#31262.
Closes https://github.com/ppy/osu/issues/31286.
Curious on thoughts about how the instant arrow fade looks on
non-classic skins. On argon it's probably fine, but it does look a
little off on triangles...
Would close https://github.com/ppy/osu/issues/31312.
Not super happy with the performance overhead of this, but this is
already a heuristic-based implementation to avoid every-frame
`.ChildrenOfType<>()` calls or similar, so not super sure how to do
better. The `Array.Contains()` check stands out in profiling, but
without it the indicators can collapse *too* eagerly sometimes.