So the default behavior of the screen split button on the editor is to toggle between horizontal split and no split, with vertical split being the option when you press alt. But when I alt-tab to another window, when I come back, that button (without pressing anything else) now toggles between horizontal and vertical, with no option to go back to no split. In order to reset the button back to the default, I have to alt-press it, which toggles the split to whatever the button was at the time, but then turns the button back into the no-split version. Alt-tabbing again doesn’t fix the problem, you have to alt-press the button directly in order to reset it.
Thank You, squintlysays. This has been filed in our bug tracking system.