Expected: I can expand the form. There is no flash of unstyled content.
Actual: I can't expand the form - there is no toggle (inserted in HTML or JS!)! There is a flash of unstyled content.
= Developer notes
Fixed by mw.loader.using('mediawiki.page.ready')
Seems that code is needed on this page.
mediawiki.page.ready is a default module for all skins EXCEPT Minerva which purposely disables it. It has multiple responsibilities (including collapse/expand behaviour on article pages - see T111565)
As a temporary measure, we'd want to load mediawiki.page.ready on the special page.