Glitchy banner drop shadow in iOS Safari

[asmeurer]

In iOS Safari, when I go to, e.g., https://pradyunsg.me/furo/kitchen-sink/blocks/, the drop shadow of the top banner glitches in and out as I scroll. Sometimes when I stop scrolling it is there, and sometimes it is not. I can record a video showing what happens if you'd like.

This might be a bug in mobile Safari as I cannot reproduce it in desktop Chrome, Safari, or Firefox. However, even on desktop, I noticed that the drop shadow is not there when I initially load the page, but when I scroll down it appears, and doesn't disappear again when I scroll back to the top of the page. It's not clear to me what the banner drop shadow behavior is supposed to be in any case.

I am on the latest iOS and macOS.

[pradyunsg]

Bundling that in with #681