Closes #47307

In markdown, the outline item for a heading actually ends on the same
row as the next outline item (see #47307 for the visual glitch). This
change makes it so that the end row calculation accounts for that, and
you don't have two different display points where content is potentially
scrolling out of view. Fixed visually:



https://github.com/user-attachments/assets/e0a7e57a-fd41-4484-87e9-ac3e51d0b927


Release Notes:

- Fixed a visual glitch with sticky scroll headers in markdown