Thanks for the feedback @Arkshine 
We have decided to call this experiment a viable success. We will remove it for now, but the possible outcome at the moment is for Discourse to have a standalone route for logging in to the forum.
When we have a distinct route, this will solve things like the back button not working, scrolling appearing when content is not visible, and give us a bit more control over the CSS as it will be standardized, rather than via this component.