Product page should keep selected variation after Add to cart

I'd make this a feature request and not a bug... The solution I can think of would be to alter the form action to append the ?v query parameter.
Also, feature requests should be opened against the 3.x core version.

I added a bit of logic in the AddToCartForm class in submitForm(), I compare if the selected variation is the same as the default variation, if not add a query parameter (?v) with the variation ID to the URL and create a new Redirect in the form state.