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A) You are completely dismissing that SteemConnect is not working either.
B) Keychain works for every other proposal vote on the page, except the return proposal.

Could you perhaps look into the possibility that maybe I'm trying to report a legitimate bug here and not looking for general support?

EDIT: Chrome (latest) has the exact same issues as Firefox. No popup appears when attempting to vote for the return proposal via SteemConnect or Keychain

Hmmm.. I'll update my Chrome and Firefox and test it out one more time. If popup doesn't work, then one of the potential issues can be that javascript simply turned off in your browser.

UPDATE: I check Chrome, Firefox and even dead Internet Explorer. Popup works everywhere. Try to clear your cookie. Keychain indeed didn't work for returning proposal though, so thanks for catching this one. Already pushed an update. Should be working right now.