OAuth2 implicit grant (token flow) does not work in IE/.Net Winform WebBrowser

"why Dropbox does the redirect with fragment and not the regular query (?) tag when doing the token flow?"

That's how OAuth 2 works. As to why OAuth 2 works that way, it's to prevent the token from being sent to the server. The query string is part of the URL that's sent to the server, while the fragment is not. The token flow is for client-side apps, so the fragment is a way to communicate the token to the app without having it leak elsewhere.

Nekobul, the behavior you describe sounds different from either of the issues already raised in this thread. It sounds like the Navigated event is called but the URL you see is lacking the URL fragment. Is that right?

One guess: if you're using https://www.dropbox.com/1/oauth2/redirect_receiver as your redirect URI, that page removes the fragment of the URL after a short period (e.g. to keep someone from copy/pasting the URL and leaking their token somewhere). Is it possible that Navigated is firing multiple times: once for the initial load of /redirect_receiver (with the URL fragment) and then once after the fragment is removed? Try logging/alerting in Navigated to see if it's being called more than once. The first call is the only one that will have the fragment.