splitX bugfix test + simplification

Hello up there.

In the process of going through b code I've noticed that splitX bug fix (commit 5c732b36) handling logic was not covered by tests for edge cases and that this edge-handling logic itself can be simplified. Please find patches which add corresponding test and do the simplification attached.

The simplified logic will be handy for upcoming work.

Please see details in individual commit messages.

Thanks beforehand, Kirill