Feedback from Waldir

Then what about adding it in the template with display:none, and unhiding it in the gadget’s CSS? That would still cause a flash after first paint as gadget CSS is loaded with JavaScript, but the hiding is easier and less error-prone to remove when the gadget becomes default, and it’s also faster as we can save the AJAX request loading the necessary MediaWiki messages. (The flash could be resolved by using a peer gadget, but I don’t think it’s worth the effort for this temporary solution.)

Sure, that sounds reasonable. Oh, and I didn't know about that peer thing, that sounds interesting – but I agree it's not worth the effort just for this.