Here's the issue: * Attempt to login via /users/login * Sometimes, the old-style login form will appear and remain for awhile * Start to fill out the old-style form... * At some point, the BrowserID blurb and button will suddenly replace the old-style form I suspect this is part of the AJAXification process we did to the Sign In button in bug 721171. But, it feels like a confusing & jarring experience, especially if the AJAX request takes awhile for whatever reason. FWIW, there's been at least one mention / complaint about this in #identity as a BrowserID issue that's actually an MDN issue.
I'd suggest we don't progressively enhance from old-login to BrowserID on this page, especially not if it waits on the completion of an HTTP request in AJAX. I'd say show the BrowserID blurb for enabled locales and show a loading spinner until the form has loaded. Showing and replacing the old-style login just seems like we're giving it and then taking it away, which was frustrating.
Someday I'm going to write up a MDN BrowserID summary of all the issues we've addressed/fixed in subsequent sprints after our initial launch of BrowserID.
Only show sign-in button after it's wired.
verified fixed https://developer.mozilla.org/en-US/users/login