I think the White background that pops up is jarring. I persoanlly think it would be better to preserve the users context and show the unlock modal and list of logins in a context aware way so that you can still see the web page behind.
Apple's demo of App Extensions showed a good example of showing the app extenstion as a smaller than full screen cantered modal. Implementing this in using the new iOS 8 API is easier than ever. When iPhone 6 comes out with larger resolution screen I think this will be even more of a reason to do this. At the moment on the Beta when I open the extension most sites just have one login so there is a huge amount of white space.