Chrome Version: 69.0.3452.0 (Official Build) dev (64-bit)
OS: Ubuntu 18.04

What steps will reproduce the problem?
(1) Load https://bugzilla.mozilla.org/attachment.cgi?id=8979827
(See https://github.com/w3c/csswg-drafts/issues/2794#issue-333365457 for an overview of this testcase -- basically, it's got a sticky positioned element which is left behind in the overflow area of a scrollable element).

(2) Try to move the scrollbar up and down.

and/or:

(3) Try resizing your window horizontally.

What is the expected result?
 - When you scroll to the top of the scrollable area, the scrollbar should probably go away, because nothing is left to create overflow and cause the scrollbar to be present. (The only thing left causing overflow is the sticky-positioned element, which moves up as you scroll up.)
 - When you resize the window, nothing should dramatically change.

What happens instead?
 - When you scroll to the top of the scrollable area, Chrome leaves the scrollbar around - it incorrectly thinks there's something left to be reached with the scrollbar.
 - When you resize your window horizontally, Chrome suddenly removes the scrollbar. (It flushes some pending relayout work that it otherwise doesn't ever get to, I think.)


It's unclear to me precisely what should happen with the scrollbar in this testcase (and I filed a spec issue: https://github.com/w3c/csswg-drafts/issues/2794 ) -- but both above-noted Chrome behaviors seem illogical & buggy to me. Hence, filing this bug.