This is new as of Safari 13 on OSX.
iFrames seem to trap/capture scroll events and prevent the document to scroll (trackpad/touch input).
Page scrolls normal as if the iFrame were an image.
Page is not scrollable when the (touch) input pointer is above the iFrame (green rect in example).
Using the mouse wheel works. So "just" trackpad is affected.
I can reproduce. Possibly a latching issue.
EventHandler::platformCompleteWheelEvent() finds the non-scrollable iframe as the thing to latch to and thinks that we started scrolling on it.
I think the bug is that scrolledToEdgeInDominantDirection() doesn't consider scrollable="no" iframes.
Created attachment 387590 [details]
The commit-queue encountered the following flaky tests while processing attachment 387590 [details]:
editing/spelling/spellcheck-attribute.html bug 206178 (authors: email@example.com, firstname.lastname@example.org, and email@example.com)
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Comment on attachment 387590 [details]
Clearing flags on attachment: 387590
Committed r254497: <https://trac.webkit.org/changeset/254497>
All reviewed patches have been landed. Closing bug.