Preferences > User Interface > Open Documents As Tabs was stored but did
nothing. With the box cleared, each document opened or created afterwards
opens in a floating window of its own, at the next cascade place, as
Window > Arrange > Float in Window floats it. Documents already open, and
documents open when the app starts, keep their tabs; checking the box
again leaves floating documents in their windows.
The engine records the preference on each document as it opens
(DocState::opened_as_tab), and the UI finds new documents by uid, so a
command batch that closes and opens documents, or changes the preference
between opening them, places each one as the preference was when it
opened.