When splitting editors, ensure that the header is right-aligned.
---
As shown in {T100141} splitting an editor creates a new one with its header centered rather than left-aligned.
This patch is the **minimum** partial revert of {D6505} that fixes this issue. This code section change was from {D10709}, which was attempting to fix {T72392}. As that fix was only to fix a problem related to vertical scrolling, this revert is only to bring back the code related to horizontal scrolling.