I was recently making an app using the SE API's client-side auth flow. What I noticed is that there was this
Unable to post message to http://localhost. Recipient has origin http://localhost:5000.
So after doing some debugging, I found that https://stackexchange.com/oauth/login_success
page, the script which notifies the original page that OAuth was successful has these lines:
$(function () {
var domain = 'http://localhost';
var proxy = 'http://localhost:5000/static/proxy.html';
It looks like the 'domain' is incorrect (as it doesn't have the port), from what I can tell this looks like it is extracting it incorrectly from proxy.
I know that running my server on :80
would fix this but it would be nice to not need to do that.