Comment by h2s

     > they thought for some reason they owned everything
     > I created, at any time, ever (sorry, no)

How common is this? I have yet to encounter it, but if I did I'd consider it an enormous red flag. It sends some very strong messages:

    - We lack the respect for you to even be subtle about
      leveraging the asymmetry of our power relationship in
      order to try to fuck you.
    - None of your potential future colleagues here are likely
      to be passionate about their craft, because people who enjoy
      making things of their own don't sign our contract.

I'm aware that employers are often willing to compromise about things like that during negotiations, but by that point the message has surely been sent.