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

Closes #53199

Mathematically, `floor(A) + floor(B) != floor(A + B)`. The original code
calculated the hovered row by applying `.floor()` to the scrolled offset
and local offset separately before adding them together, which
incorrectly dropped fractional sub-pixels and caused an off-by-one
targeting error.

Release Notes:

- N/A

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Co-authored-by: Anthony Eid <anthony@zed.dev>