Fix random selection not showing selection when all groups are collapsed

[peppy]

Closes #36365.

This reverts a previous fix (#35163 plus follow-up regression fix) and applies a hopefully more permanent fix which doesn't affect other scenarios.

I also did some refactoring, which is in the last commit. It can be skipped or applied separately as preferred (but without it I have trouble understanding the fix I made here).

Note that I'm still dealing with surrounding flaky tests and only pushing this PR out to see how production CI responds. It should be considered a functional RFC.

[bdach]

I don't understand this diff at all.

8860eec replaces a HandleItemSelected() call with inlining the non-problematic part of it that doesn't expand the group. aside from the fact that it's for whatever reason not checking BeatmapSetsGroupedTogether before trying to expand sets that may not exist, I get that part.

but then... 46b2d53 reverts to using HandleItemSelected()? and instead adds some is GroupedBeatmap check that is presumably load-bearing here, but I don't know why? is this a null check? is this a type check? no idea. there's also a group expansion operation at the end that was seemingly straight-up removed? is that load-bearing / the moved fix? if not, where did it go?

[peppy]

and instead adds some is GroupedBeatmap

This part was just tidying up. The important part is the removal of the final setExpandedGroup call via the refactoring.

The final diff makes more sense to me because we're calling just HandleSelectedItem in all cases (which can be rationalised as "running even in the case of model equality for visual resync purposes"), and it's always working as expected.

[bdach]

I was unconvinced so I went fishing and found an apparent regression:

1. start with a grouping mode set and beatmaps grouped by set
2. select any beatmap
3. collapse its enclosing group
4. filter it out
5. clear the filter
6. expand the group
7. the set panel of the selected beatmap will be collapsed and will not respond to attempts of expanding it

doesn't appear to happen on master.

Screen.Recording.2026-01-21.at.09.52.48.mov

appears to be failing on the following guard:

osu/osu.Game/Screens/SelectV2/BeatmapCarousel.cs

Line 494 in 46b2d53

if (selection.Group == ExpandedGroup)

[peppy]

I haven't made a test for your provided case yet, but I've hopefully fixed it.

Will add a test tomorrow. I'm pretty happy with the latest diff.

[peppy]

Added test coverage of failing scenario.

[bdach]

can't seem to break this anymore. at the same time I also expect someone to find some breakage and this to require another fix on top, but we can add unit tests ad nauseam until hopefully a stable equilibrium is reached 🤷