As far as my personal opinion, I think we introduce complexity and increase confusion by trying to automatically sign a user in from the signup page. I have no evidence other than bug reports to back up my claim, but the reports we have are that people are surprised we automatically try to sign them in.

Is the confusion/unease mitigated by automatically switching to the `/signin` page, prefilling the data, and trying to sign the user in from there? I'm having a hard time imagining it does. Switching screens could cause more confusion and bring up questions like "hey, why did the screen change on me?"