This was reported in https://github.com/ppy/osu/pull/28474, albeit the
code changes proposed there did not fix the issue at all.
See 8b6385f7d0 for demonstration of the
crash scenario. Basically what is happening there is:
- The starting premise is that there is a spinner placement active.
- At this time, a drag selection is started via the timeline.
- Once the drag selection finds at least one suitable object to select,
it mutates `SelectedItems`.
- When selection changes for any reason, the `HitObjectComposer`
decides to switch to the "select" tool, regardless of why
the selection changed.
- Changing the active tool causes the current placement - if any -
to be committed, which mutates the beatmap.
- Back at the drag box selection code, this causes a "collection
modified when enumerating" exception.
The proposed fix here is to eagerly commit active placement - if any -
when drag selection is initiated via the timeline, which avoids this
issue. This also appears to vaguely match stable behaviour and is sort
of consistent with the logic of committing any outstanding changes upon
switching to the selection tool.
Alternate method is to expose a `SnapDistancesChanged` event in
`IPositionSnapProvider` instead, but I chose this way as an analogue to
`IBeatSnapProvider.BeatDivisor`, which might even make sense to be
exposed as `BindableBeatDivisor` instead of caching that separately.
