Fix sticky header in last buffer of a multibuffer (#26944)

This also simplifies our code to stop generating a last excerpt boundary
that we always ignore.

Closes #ISSUE

Release Notes:

- N/A
25772b8777Conrad Irwin committed on 3/17/2025, 6:39:57 PM· committed by GitHubparent94b6380
5 files changedLine totals unavailable