In Apple Mail on my Mac running OS X 10.13.2 (High Sierra), I'm not getting the split view in full screen mode. No matter what I do, replies and new messages bring up a new window that hovers over top of everything instead of appearing in a split fashion off to the right. I am viewing in full screen mode. I do have the option checked in Mail's preferences to "Prefer Opening Messages in Split View When in Full Screen". What could be wrong? --- Post Merged, Dec 6, 2017 --- I discovered that there is a setting in Mission Control that also needs to be enabled. It's the one about displays having separate spaces.