Hey there, hope you’re well !
It seems that Safari (Mac) has some problems, if you leave the fullscreen-mode using your lightbox.
Safari 7.1.6 and the last version of safari 6.
The calculation of the right windowsize seems to fail, after leaving fullscreen (just 100px).
It works well in Chrome, Opera and Firefox. On IE it seems to work well, too.
I’ve made a Screenvideo for you with your own demo, so you can be sure that no custom changes cause the problem. We’ve got exactly the same results on our website. It happens with 2 different MacBooks. It happens also, if all extensions are deactivated.
Here you can see what I mean, but please don’t publish the link (our test-environment):
Thank you and best regards,