iOS 12 is not preventing the scrolling of a document under fixed positioned overlay

Live demo: https://whimsical-surfboard.glitch.me/

The expected behavior of this demo is: scrolling of a document is prevented because it's under the overlay.

The live demo works as expected in desktop browsers, Android, and iOS 11. But iOS 12 seems to behave differently and scrolls a document.

Here's the minimal styles:

body {
  overflow-y: hidden;
}

#backdrop {
  position: fixed;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  background: rgba(0, 0, 0, .2);
}

Source code is here, click "remix this" to fork it: https://glitch.com/edit/#!/whimsical-surfboard

The question is: how to make iOS 12 behave the same way as other browsers, i.e. prevent the scrolling.

from iOS 12 is not preventing the scrolling of a document under fixed positioned overlay