A small test ground to try to see how Facebook does its modal-ish interface for creating new post.

• We really need to know where the scrollable, parent container is. With Facebook, they have the whole body as a scrollable element. So the modal backdrop could be added with position: fixed.

• For complex, gridded layouts and such, it will get quite tricky. Because using a position: fixed modal backdrop will mean that the background is not scrollable, but the modal content is scrollable. Using a position: absolute modal backdrop is possible, but will need a parent that is position: relative, to properly wrap over the whole scrollable content.

• We can use IntersectionObserver to detect when user has scrolled away instead, which is a lot better than getBoundingClientRect. Only problem is, IntersectionObserver is not supported in IE11, and will require a polyfill.