renderFloatPanel/propsFloatPanel set the surface, overriding only when
given (otherwise the outer surface flows through unchanged), so the
nearest explicit surface wins.
The top provider in a tree (no enclosing float) automatically owns a fresh
coordinating root; nested providers inherit it. isolate forces a new
root even when nested (a sub-scope).
Configures the float system for a subtree.
renderFloatPanel/propsFloatPanelset the surface, overriding only when given (otherwise the outer surface flows through unchanged), so the nearest explicit surface wins.isolateforces a new root even when nested (a sub-scope).