Can't split screen into multiple panes; selection is ignored.

Make sure you own the Advanced Feature Set to enable the split-screen feature. You can confirm this by going to Settings, then About. There should be some mention of the Advanced Feature Set there.

If the AFS is active but the split screen feature still doesn't work, you might have Maximize Pane enabled. Bring up the app menu bars by right-clicking on the screen, and select Maximize Pane to toggle this feature off. Try splitting the screen again and it should work.