Ah I figured it out!
When I toggle ‘Always keep composer in bound’ to ON, when I open up Composer in ‘float mode’, I cannot drag it off the screen.
That is cool (and the empty padding around the edge of the screen gives the feature a nice blocky ‘feel’), but I think there is still a bug where, if that feature is toggled OFF and you try and drag Composer to another screen it disappears - I documented that behaviour here:
PS I am still not sure what some of those other Composer settings mean 
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