Each SectionStackSection has a a collection of items, you can get them by getItems.
It seems that all these Canvases are hidden when you click and the section collapse.
By using the console I saw by Eval isc_SectionStack_O.members that you have in this array the header and the different HTMLFlow of the section.
So I think by reading the "visibility" attribute of the second member of the array you get the status you want.
I checked that with the Console:
Eval: isc_SectionStack_0.members[1]
and look at the visibility attributre when you collapse or not
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