This is a problem I've encountered too, people are unable to login to a Google account when using insecure Webview browsers... which includes; Messenger, Facebook, Instagram... etc.
But Gmail Webviews allow Google logins though :/
But Gmail Webviews allow Google logins though :/
You could try creating an issue in the Cloud Identity issue tracker (Cloud Identity is Google's API for letting websites have a "Login with Google" thing): https://issuetracker.google.com/issues/new?component=522910&...