So I use a nice Chrome plugin for testing responsive layouts called “Dimensions Legacy”, and it puts your content in an iFrame and lets you resize the frame without having to resize your browser (good for inspecting!), HOWEVER, wordpress has added a Frame showing restriction into their default security that won’t allow wordpress to render in an iFrame anymore!
I tried a few things that were meant to make it work, like commenting out the header hooks and even the function that inserts the X-Frame directive, but nothing seemed to work. Even after a reboot!
Finally I found a nice solution (in StackOverflow), that lets you invalidate the X-Frame SAMEORIGIN directive, by adding your own nonsense one that makes the browser ignore them both!
In my page template I added:
<?php header('X-Frame-Options: GOFORIT'); ?>