When overscroll-behavior is defined on iframe document element, the scrolling should not chain to the parent of that iframe.

Chrome behavior is broken here. We seems to be ignoring the overscroll-behavior property.

See attached testcase (based on Firefox testcase) with two types of iframes
A- iframe where document element specifies overflow:scroll
B- iframe where document element does not specify an overflow. Therefore the frameview scrolls instead.

I believe in both cases the behavior should match that of the Div element.

## Side note

Our broken behavior is inconsistent between main and impl thread scrolling.

Main scrolling: Both A, B and broken.
Impl scrolling: Only A is broken!


See also: 
- related Firefox bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1494440
- original issue report: https://github.com/WICG/overscroll-behavior/issues/31

 

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