Further fine tune scroll locking on iOS

[RobinMalfait]

This PR fixes an issue that I only noticed in a production build and not in the local development
version (potentially related to strict mode?).

However, we are trying to scroll lock the page on iOS, a way to do this is to add
overscroll-behavior: contain; and call event.preventDefault() in the touchmove event. While
this works great, we have to account for scrollable elements inside the allowed containers (e.g.:
a scrollable table in a dialog).

We did this by checking whether or not the event.target or any of its parent were scrollable. The
heuristic we used was two-fold:

1. Does the computed style have overflow: auto | scroll;, overflow-x: auto | scroll; and overflow-y: auto | scroll;
2. Or, is there an overflow happening element.scrollWidth > element.clientWidth or element.scrollHeight > element.clientHeight

The issue is the first step, if an overflow property exists but there is no overflowing happening
then the overscroll-behavior for some reason doesn't have any effect and it will result in the
main page being scrollable.

Purely relying on step 2 seems to fix this issue.

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