You always had to be a little careful about what you had open in order to be able to use split screen because of the limited width - so, for instance, you could never have the inspector, binder and editor, or a vertical split, because that wouldn’t leave enough screen space. In Scrivener 3, though, the editor requires a larger minimum width which complicates things. The problem is that in Scrivener 3, the minimum width is set to the minimum required for both editors in a vertical split, rather than only a single editor - this is because of the complications of calculating the necessary width for all scenarios, with the added complexity of Copyholders. For the Scrivener 3.0.1 update, this has been addressed, so that the editor no longer requires enough space for both editors at all times and can be shrunk down further again.