This is probably not a great idea but... the VisualEditor extension would like to load the built-in Parsoid extension from vendor/wikimedia/parsoid iff LocalSettings.php didn't already load a version of Parsoid.
But the way ExtensionRegistry::loadFromQueue() works, the custom extension callback is called only *after* all the extensions are loaded, so even though Parsoid gets added to the queue for loading, it is never loaded.
We could move up the callbacks so that this works. Or do some more evil hack...