An extension shows blank or throws "require is not defined" in Hyvä, can you fix it?

Yes. That happens when a Luma-only extension injects RequireJS markup into a Hyvä page that doesn’t load RequireJS, so its require([...]) call throws Uncaught ReferenceError: require is not defined and the feature renders blank. The fix is one of: install the vendor’s Hyvä compatibility module, write a small compat bridge that re-implements the widget in Alpine, or disable the Luma frontend output and replace it with a Hyvä-native equivalent. We’ll tell you which is cheapest in your case.