DropdownWidgets at the end of a PageLayout are very hard to use, because they are truncated to not overflow the page. This especially happens on Special:APISandbox. The screenshot shows https://de.wikipedia.org/wiki/Spezial:ApiSandbox#action=query&format=json&prop=categories&cldir=ascending
I don't know whether it is possible to specify an alternative $overlay in this case. If it is, this should be fixed in the JS code for Special:APISandbox, if not, OOjs UI should be fixed to allow this.