[URL removed per my bosses request] – please see screenshot in first comment to see the bug that was occuring.
Check that site out. For some reason, sometimes when loading the login page I get a dark stripe behind the login box (this only shows up in Chrome and Safari). If you don’t see it, try Shift + Refresh a couple of times. Clicking inside the form anywhere or trying to inspect the elements seems to remove this dark box (usually, not always). I assume it is because I have an overlay that is showing on the page when trying to login. However, the overlay is set to display: none, so it should not be possible to see it. This makes the page look really ugly upon loading, so it’d be awesome if I could remove it. Any help is greatly appreciated!
So I copied your CSS and HTML and made a local version to solve this.
Here is what I did to fix your problem.
A: I set an explicit height of 100% on your body tag (to prevent any cropping or content)
B: Your margin top of your Section element is being merged or inherited (not quite sure what the term is) by your Body element. so I set margin-top:0; on your section.login-form and put it as “padding-top:45px;” directly on the body.
in summary this is the CSS I used
I didn’t include all your .login-box CSS but the pertinent line is 210.
Let me know how that works for you.