That only works for extensions that want to show their content in a separate overlay layer from the page. If the extension wants to show its content inline with

That only works for extensions that want to show their content in a separate overlay layer from the page. If the extension wants to show its content inline with the page's elements, pushing the page's elements out of the way and freely flowing with the page's elements, then that doesn't exactly work. An extension can stuff its UI within an iframe that the host page can't manipulate, but that does come with some UI limitations.