When the infinite scroll is enabled and the screen is less than 1200px, this happens:
The widget area and footer comes in between the posts. I honestly can't figure out the solution for this problem.
Confirmed. I'm working on a fix now.
Fix Jetpack Infinite Scroll when browser width < 1200px. See #69.
Fixed in latest development copy. Will go out in the next update. Thanks for reporting this @manishsuwal! :)