Closes https://github.com/ppy/osu/issues/21794.
I'm not actually super sure as to what the exact mode of failure is
here, but it's 99% to do with working beatmap cache invalidation. Likely
this can be even considered as another case of
https://github.com/ppy/osu/issues/21357, but because this is a one-liner
"fix," I'm PRing it anyways.
The issue is confusing to understand when working with the swap scenario
given in the issue, but it's a little easier to understand when
performing the following:
1. Have a beatmap set with 2 difficulties. Let's call them "A" and "B".
2. From song select, without ever exiting to main menu, edit "A". Change
the difficulty name to "AA". Save and exit back to song select; do
not exit out to main menu.
3. From song select, edit "B". Change the difficulty name to "BB". Save
and exit back to song select.
4. The difficulty names will be "A" and "BB".
Basically what I *think* is causing this, is the fact that even though
editor invalidates the working beatmap by refetching it afresh on exit,
song select is blissfully unaware of this, and continues working with
its own `BeatmapInfo` instances which have backlinks to
`BeatmapSetInfo`.
When editing the first of the two difficulties and then the second,
the editing of the first one only invalidates the first one rather than
the entire set, and the second difficulty continues to have a stale
reference to the first one via the beatmap set, and as such ends up
overwriting the changes from the first save when passed into the editor
and modified again.
It should not run in multiplayer. Even if we wanted to allow touch-only
playlist items at some point, the current behaviour of multiplayer song
selects with respect to touch device mod is currently just broken.
This was originally set high because of performance concerns. We have
since improved things quite drastically. Even with a very low debounce
my song select is amazingly smooth.
This is about as low as we can got unless we want chaos when key
repeating left/right arrows (it's not as bad as you think, maybe worth
testing during review and see if it feels more expected?).
Of note, stable has key repeat disabled on left/right arrows and zero
debounce.
Addresses concerns in https://github.com/ppy/osu/discussions/24916.
