The split button appears to be smart in that it remembers the last spit type you used, so the next time you click it, it will create your (presumably) favourite split type. Wonderful! Slight bug, though.
These steps will reproduce an error in that behaviour:
- Open a horizontal split via keyboard shortcut (Cmd-Shift-Opt A)
- The split button in the header should reflect your preference.
- Change split mode to vertical using shortcut (Cmd-Shift-Opt U)
- Again, split button should reflect your new preference.
- Click the split button to close the vertical split.
- Click the split button to open a split. Judging by behaviour model and the appearance of the button, it should open a vertical split, however it opens a horizontal split, while still displaying a vertical split button mode. This behaviour will persist until you Opt-click on the button to switch modes. Then it will be synchronised again.