Avoid expensive re-computation of sticky header lines (#46653)

Fixes a performance regression introduced in
https://github.com/zed-industries/zed/pull/45377

Release Notes:

- Fixed a performance regression that caused lag in large documents
024485133dMax Brunsfeld committed on 1/13/2026, 12:38:13 AM· committed by GitHubparent1c1ab65
3 files changedLine totals unavailable