Hello;
I've found out that, when using layout_options_class_select / ClassAttributeSelect, the #attributes classes are correctly populated for the 'layout' region but not for the rest.
As far as I can see, this is because the processAttributeOptionBuild() receives an empty $regions array, coming from Layout Builder. Thus, the elseif (array_key_exists($region, $regions)) { always fails and the attributes aren't populated.
Closed: cannot reproduce
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