The article flow does NOT fit in the region chain.
Named flows created from CSS have a corresponding DOM object. This object has a reference to its content elements and it can tell they fit completely into the region chain.
var flow = document.getNamedFlows()['article'] flow.contentNodes // NodeList flow.overset // boolean
A named flow is represented by a
NamedFlow object. The
document.getNamedFlows() method returns a
NamedFlowCollection object – individual named flows can be retrieved either by index or by name from this collection.
contentNodes property references a
NodeList of elements which are collected into the named flow.
NamedFlow object has a boolean property called
overset. If the flow's content fits completely in the region chain then the value of
overset is false, otherwise it is true.