The page http://output.jsbin.com/iYEmaTUF/1 didn’t look like the screen vid of it loading in for me using Firefox. No where near as “jumpy”.
I do notice some differences between his demo and mine
mittineague.zip (106.7 KB)
His has quite a few media query points, mine has but one.
The nav is between the main and the aside, mine is in header.
The JavaScript transitions may be causing some buggyness*, mine is a simple display block none toggle.
*Repro steps
note left edge of contaner
narrow view-port until hamburger shows
open menu
widen view-port
The left edge “clings” as the view-port widens and is now further to the right than it was when the page first loaded.i.e width values are getting foobar for some reason.