# Objective

Fixes #62524

This bug was introduced in a regression from #61487, which fixed
horizontal autoscroll for multi-row selections (word wrap off) by
computing target_left/target_right from the selection's actual start/end
instead of just head.That fix changed target_left/target_right from
always being a single point (head's column) to spanning the full
selection width. This tripped an existing guard (if target_right -
target_left > viewport_width { return None; }) whenever a selection was
wider than the viewport, previously dead code, since
target_left/target_right were never far apart before #61487. The
function now bails out before adjusting scroll at all in that case.

## Solution

In `autoscroll_horizontally`, compute the selection's span width per
row. If it exceeds the viewport width, fall back to tracking just head
(the pre-#61487 behavior) instead of the full start/end span. If it
fits, keep using the full span so the #61409 fix is unaffected.

## Testing

- Did you test these changes? If so, how?
- Tested manually in-app on macOS: selecting a long line with
cmd+shift+end now scrolls correctly.
(NOTE: I used Claude to write these unit tests for me)
- Added test_autoscroll_horizontally_long_selection_tracks_cursor:
selects a 250-character line in a narrow viewport and asserts the scroll
position moves to follow the cursor, instead of staying frozen at 0.
- Added
test_autoscroll_horizontally_fitting_selection_reveals_full_span:
selects a short span that fits within the viewport and asserts the full
span is revealed, confirming the #61409 fix still holds.
- Ran the full editor test suite locally (cargo test --package editor
--lib), all passing.

## Self-Review Checklist:

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

---

Release Notes:

- Fixed horizontal scroll not following the cursor when selecting a line
longer than the viewport width

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Co-authored-by: Kirill Bulatov <kirill@zed.dev>