This is an odd little bug I've been having and I can't, for the life of me, find what's causing it!
What I noticed is this - The first time I ever open the nav menu on my site, it doesn't perform the smooth transition from off the page.
I discovered this happens on both Chrome and Safari on my iPhone.
Further testing however, shows that the transitions works perfectly fine in IE11 and Firefox.
Anyone know what might be causing this?