This solved it for me. Developer Mode was off on my new iPhone. The phone I had migrated from was running iOS 15, which predates the Developer Mode setting.

So... no fix for those of us still on Catalina?

@arafath_mk, I don't think setting the heights to 100vh really fixes it. In my sample code, when the URL and nav bars are present, "100vh" extends the div so that it's underneath the nav bar. The div is taller than the available screen space, and you can actually scroll up and down. What we want is to have the div size itself to the available screen size, regardless of orientation. Thanks for the suggestion, though.