This crash occurred when the left hand side (LHS) editor in the split
diff retained a stale shared scroll handler after the split editor
unsplits. I added an invariant check to our randomized companion display
map tests to prevent regressions here and explicitly make LHS editor set
a native scroll anchor when unsplitting.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fix crash that could occur when unsplitting a side by side diff view