Your hero section is height: 100vh . Big background image, headline centered, a CTA button pinned near the bottom. You check it in the browser, check it in the iOS simulator, ship it. Then the support tickets come in: "the sign-up button is cut off." You open it on your own phone to confirm — and it looks fine. Scroll down an inch, though, and there's a sliver of button peeking out from under where the fold should be. Guess what's different between your simulator and a real phone in so...