The use of Alt key to temporarily change split button from horizontal to vertical split is not obvious until one wonders where the vertical split button went and happens to hover over the horizontal split button. Seems non-intuitive and frustrating, even after learning this. Have both horizontal and vertical split buttons or have horizontal/vertical button stay in whichever state last toggled to and used? Personally, I do far more vertical splitting, for side-by-side work, than horizontal splitting.
Scrivener 188.8.131.52 beta (19429) 20 Nov 2017
Windows 10 Pro 64 bit 10.0.15063