A quick status update here:

I landed some heuristics to disable scroll anchoring in pathological cases in Firefox a long while ago. This stopped virtually all compat issues, though it's obviously not great.

Chris and other Chromium folks have been doing work to fix Chromium issues that were causing these interop problems, and improving the scroll anchoring spec.

So I'm going to try and peek up those spec changes in Firefox and then try to remove those heuristics on Nightly, and see how it goes.

 -- Emilio

On 11/7/19 12:07 AM, Chris Harrelson wrote:
HI Emilio,

I'll follow up on crbug.com/920289 <http://crbug.com/920289>. Let's discuss there.

On Tue, Oct 29, 2019 at 3:03 PM Emilio Cobos Álvarez <emi...@mozilla.com <mailto:emi...@mozilla.com>> wrote:

    Hi all,

       10/18/19 7:19 PM, Chris Harrelson wrote:
     > Hi,
     > Another quick update: Emilio, Navid, Nick, Stefan and I met today and
     > discussed which issues are important to fix and why. We now have
    a list of
     > spec issues, and WPT tests to fix that are Chromium bugs, that should
     > substantially improve interop. Nick and Stefan will take on the
    work to fix
     > them, with the review and feedback support of Emilio.

    So, today another scroll-anchoring bug crossed my radar, and this one
    I'm not sure at all how to fix it, because there's no obvious answer
    here as far as I can tell.

    My diagnosis (for one of the pages, the one I could repro and
    reduce) is
    in here[1], but basically my current explanation is that the page
    be broken per spec, and that when it works it's hitting a bug in both
    Chromium[2] which we have an equivalent of but are just not hitting
    because in Firefox changing `overflow` does more/different layout work
    than in Chrome.

    The test-case may as well work if we change our scroll event or timer
    scheduling (see there), but that is obviously pretty flaky.

    I honestly don't have many better ideas for more fancy heursitics about
    how to unbreak that kind of site. From the point of view of the
    anchoring code, the page is just toggling height somewhere above the
    anchor, which is the case where scroll anchoring _should_ work, usually.

    I can, of course (and may as a short-term band-aid, not sure yet) add
    `overflow` to the magic list of properties like `position` that
    scroll anchoring everywhere in the scroller, but that'd be just kicking
    the can down the road and waiting for the next difference in layout
    performance optimizations between Blink and Gecko to hit us.

    I think (about to go on PTO for the next of the week) I'll add
    for pages that have scroll event listeners, and see if disabling scroll
    anchoring on a node when there are scroll event listeners attached
    to it
    is something reasonable (plus adding an explicit opt-in of course).

    I'm not terribly hopeful that the percentage of such documents is going
    to be terribly big, to be honest, but providing an opt-in and doing
    outreach may be a reasonable alternative.

    Another idea would be to restrict the number of consecutive scrolls
    by scroll anchoring to a given number at most. That would made the
    experience in such broken websites somewhat less annoying, but it'll
    also show flickering until that happens, which would make the browser
    still look broken :/.

    Thoughts / ideas I may not have thought of/be aware of?


       -- Emilio

    [1]: https://bugzilla.mozilla.org/show_bug.cgi?id=1592094#c15
    [2]: https://bugs.chromium.org/p/chromium/issues/detail?id=920289

     > Thanks all,
     > Chris
     > On Thu, Oct 10, 2019 at 2:13 PM Rick Byers <rby...@chromium.org
    <mailto:rby...@chromium.org>> wrote:
     >> Sorry for the delay.
     >> We agree that scroll anchoring has unrealized potential to be
    valuable for
     >> the web at large, and to make that happen we should be investing
    a lot more
     >> working with y'all (and if we can't succeed, probably removing
    it from
     >> chromium). Concretely +Chris Harrelson who leads rendering for
    Chrome (and
     >> likely someone else from his team), as well as +Nick Burris from
    the Chrome
     >> input team will start digging in ASAP. In addition to just the
     >> high-bandwidth engineer-to-engineer collaboration between
    chromium and
     >> gecko I propose the following high-level goals for our work:
     >>     - Ensure that there are no known deviations in behavior between
     >>     chromium and the spec (one way or the other).
     >>     - Ensure all the (non-ua-specific) site compat constraints
    folks are
     >>     hitting are captured in web-platform-tests. I.e. if Gecko
    passes the tests
     >>     and serves a chromium UA string it should work as well as in
    Chrome (modulo
     >>     other unrelated UA compat issues of course).
     >>     - Look for any reasonable opportunity to help deal with
     >>     compat issues (i.e. those that show up on sites that are
    explicitly looking
     >>     for a Gecko UA string or other engine-specific feature).
    This may include
     >>     making changes in the spec / chromium implementation. This
    is probably the
     >>     toughest one, but I'm optimistic that if we nail the first
    two, we can find
     >>     some reasonable tradeoff for the hard parts that are left
    here. Philip (our
     >>     overall interop lead) has volunteered to help out here as well.
     >> Does that sound about right? Any suggestions on the best forum
    for tight
     >> engineering collaboration? GitHub good enough, or maybe get on
    an IRC /
     >> slack channel together somewhere?
     >> Rick
     >> On Mon, Oct 7, 2019 at 2:11 PM Mike Taylor <mi...@mozilla.com
    <mailto:mi...@mozilla.com>> wrote:
     >>> Hi Rick,
     >>> On 9/28/19 10:07 PM, Rick Byers wrote:
     >>>> Can you give us a week or so to chat about this within the
    Chrome team
     >>>> and get back to you?
     >>> Any updates here?
     >>> Thanks.
     >>> --
     >>> Mike Taylor
     >>> Web Compat, Mozilla
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