Context

Since #52232, there have been a lot of problems related to panels randomly hiding and showing themselves. However, the code in that PR is pretty innocuous. It shouldn't be having that kind of effect. However, it turns out there's a bug in how GPUI subscriptions work that caused there to be orphaned observe callbacks firing spuriously. The core sequence is captured in test_unsubscribe_during_callback_with_insert, essentially you have to have two observers on the same item (SettingsStore, in this case), that both drop themselves. If the first observer adds a callback onto the same item, then the second observer will never successfully drop itself.

While in there, I also fixed an unrelated bug that @maxbrunsfeld noticed where if you have two callbacks on the same item, and an earlier callback drops a later one, then the second callback will spuriously fire.

I also added a few extra smoke tests to the subscription code, and a test capturing the observed bug at the workspace level.

Self-Review Checklist

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• N/A