Clip split buffer headers to the editor's content bounds so they no
longer paint into the horizontal scrollbar area.

Reuse the editor's computed right margin and horizontal scrollbar
visibility instead of reimplementing scrollbar detection in
split_editor_view. This keeps split headers aligned when minimap, inline
blame, or scrollbar visibility change the available content area.

Release Notes:

- Fixed horizontal scrollbars not being rendered behind buffer headers
in split editors

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Signed-off-by: Xiaobo Liu <cppcoffee@gmail.com>
Co-authored-by: dino <dinojoaocosta@gmail.com>