With new_dock_zones (and without side_edge_zones), the body of a frame or
floating group that takes no tabs is no drop target, whatever its
accepts_splits. A floating window released there stays floating and a
docked panel floats as over empty space; the outer edge band, dock columns,
new docks and drops beside other groups still dock. Without new_dock_zones
the whole canvas body still goes to the nearest of drop_edges.