Black flash before page loads on page with looping video background

I created a new site and added a single section with a looping video as the background. When the page loads, I see a brief flash of black. I've tested this with two different videos and it happens with both. There is no additional custom code on the site. I'm hoping you can help determine what's causing the issue and suggest a fix.

Another thing I noticed — on mobile devices in Safari, the first time the page loads, you have to scroll up to see the header. Is there a fix for this as well?