Facebook login window for Desktop application

I'm working on a desktop facebook application, to show facebook login window

FB = window.runtime.com.facebook;
fb = new FB.Facebook();
sessionHelper = new FB.utils.Desktop开发者_运维技巧SessionHelper(api_key)

The facebook login window doesn't show the full page height, and there's no scroll.

Also, any clue why the sessionHelper.logout(); doesn't take effect except when user restart the application?