On 9/29/19 5:07 AM, Rick Byers wrote:
On Fri, Sep 27, 2019 at 10:16 AM Emilio Cobos Álvarez <emi...@mozilla.com <mailto:emi...@mozilla.com>> wrote:

    Hi Steve,

    On 9/27/19 4:03 PM, Steve Kobes wrote:
     > Hi Emilio,
     > My recollection is that scroll anchoring was, in fact, a mess.  I
    do not
     > personally have any opinion about whether scroll anchoring should be
     > removed from Gecko.
     > We (Chrome) decided to accept some compat issues for the sake of
     > launching the feature.  This was a judgment call and could
     > have gone the other way.

    Right, my concern is that taking compat fallout with Chrome's market
    share may be acceptable, because people will likely fix their websites
    if they misbehave.

    But web developers may not take the same time to fix their site if it's
    broken on Firefox for Android, for example, which in turn drives
    users away (and you know this is a vicious cycle, the less users you
    have, the less people will care about fixing their websites in your

    That being said, more generally, I care about being interoperable /
    predictable here for web developers, and seems like that ship may have
    sailed if we need to fix some Gecko-specific issues by tweaking our
    heuristics, but Chromium / Blink doesn't change them in the same way
    (which is understandable, I guess, though I've filed spec issues for
    reasoning behind these changes, which I think would apply to Chrome as

FWIW, I agree with this principle. I'm sorry you've had to do a lot of compat work on this Emilio. Are you saying you've found many cases where chromium's behavior doesn't match the spec / web-platform-tests and the different is relevant to real-world website compat (forcing you to invest in "bug-for-bug compatibility")? That would definitely make me very sad. Or is the issue more about compat with sites which have UA-conditional behavior (either explicit or implicit based on some other Gecko/blink difference?).

Well, part of it is that. The initial implementation took a lot of just figuring out what Chromium was doing rather than implementing the spec, because the spec had clear issues (like referencing the DOM rather than layout stuff).

Some of them like [1] were pretty obvious and were caught during our initial implementation of the feature. Others like [2] Ryan probably found by testing Chromium's behavior.

Some other still pretty significant behavior differences were only caught later by me and people finding compat issues in the wild, like [3]. I was sad that the spec reflected absolutely nothing like what Blink implements. For this issue in particular, Blink roughly uses "whatever inherits from LayoutBox can be an anchor", which is obviously not something that you can reasonably spec, and definitely not "block boxes and text", which is what the spec said.

Those are off the top of my head, Ryan probably has more examples.

IMHO In general, either an initially chromium-only feature is valuable enough that we should continue to invest as necessary to achieve interop with other engines when they implement (eg. adding web-platform-tests and improving the spec for the inevitable cases that appear with a second implementation), or we should decide the feature isn't worth the cost to properly support on the web at large and remove it from chromium.

Steve is the expert and can probably elaborate on details, but IIRC the real world web compat constraints of scroll anchoring ended up requiring a number of tough tradeoffs. If you're learning about new web compat constraints, then it's entirely possible that the cost/benefit equation is now different and we should be re-evaluating whether it still makes sense to keep scroll anchoring in chromium. Like David I like the feature - but only to the extent that it works alright for most of the web as it exists today, and developers can reliably reason about it (eg. by replacing any heuristics designed under the constraints of web-compat with explicit APIs).

Can you give us a week or so to chat about this within the Chrome team and get back to you?

Thanks, and sorry again for the frustration. When we ship a feature first in chromium, it's always our intent that subsequent compatible implementations should be MUCH easier to ship (it's one of the main reasons we invest so much in web-platform-tests).

Sure, no worries, and thanks for the reply.

 -- Emilio

[1]: https://github.com/w3c/csswg-drafts/issues/3480
[2]: https://github.com/w3c/csswg-drafts/issues/3319
[3]: https://github.com/w3c/csswg-drafts/issues/4247

       -- Emilio

     > On Fri, 27 Sep 2019 at 09:09, Emilio Cobos Álvarez
    <emi...@mozilla.com <mailto:emi...@mozilla.com>
     > <mailto:emi...@mozilla.com <mailto:emi...@mozilla.com>>> wrote:
     >     And, to be clear, we _can_ fix these compat issues, some way
    or another.
     >     One thought is to limit the amount of scroll adjustments
    without user
     >     scrolling or stuff like that, which would prevent the "you
    get stuck on
     >     the page".
     >     Making anchoring opt-in rather than opt-out is another
    option, but that
     >     defeats most of the purpose of the feature, I guess.
     >     See also some of the Chromium docs on the compat issues they
     >     and how were they trying to fix them before adding the
     >     "layout-affecting-property changed" heuristic, which is what
    is on the
     >     spec right now and what they implement.
     >     I just think that these are very hacky heuristics that are just
     >     going to
     >     bring a lot of compat pain and developer confusion.
     >     It doesn't help that all these things can break or not
    depending on the
     >     speed at which the user scrolls, the amount of scroll events
    that the
     >     user dispatches, the timing of these events relative to other
     >     events, etc...
     >        -- Emilio
     >     [1]:
>  <https://docs.google.com/document/d/1nQAO4MYCDMn0rTkn_-WI6gjumk3Qi2Bn-MGuB3NlVxE/edit <https://docs.google.com/document/d/1nQAO4MYCDMn0rTkn_-WI6gjumk3Qi2Bn-MGuB3NlVxE/edit>>
     >     On 9/27/19 2:23 PM, Emilio Cobos Álvarez wrote:
     >      > Hi,
     >      >
     >      > (cc'ing webkit-dev@ and blink-dev@ in case they have
    feedback or
     >      > opinions, as WebKit is the only engine which does not
     >     scroll
     >      > anchoring, though I don't know if they plan to, and Blink
    is the
     >     only
     >      > other engine that does implement it. Please reply to
     >     dev-platform@ though.)
     >      >
     >      > TLDR: Scroll anchoring is really a mess.
     >      >
     >      > I didn't do the initial implementation of the feature in
     >     but I've
     >      > done a ton of work over the last few months to fix compat
     >     in our
     >      > implementation (see all the bugs blocking [1]).
     >      >
     >      > At this point, our implementation is mostly compatible with
     >     Blink, but
     >      > even with a bug-for-bug compatible implementation, we did
    get compat
     >      > issues because of different content being served for different
     >     browsers,
     >      > or because our anti-tracking protections changing the final
     >     content of
     >      > the page slightly ([2] is an example of bug which only
     >     with
     >      > ETP enabled only, but whose reduced test-case renders the site
     >     unusable
     >      > in Chrome as well).
     >      >
     >      > If you hit one of the broken cases as a user you think the
     >     browser is
     >      > completely broken, and the site is just unusable.
     >      >
     >      > I've fixed those by tweaking the heuristics Gecko uses.
    Those extra
     >      > heuristics have also caused other compat issues, like [3],
     >      > today, which will require other adjustments to the
    heuristics, etc...
     >      >
     >      > On top of that, the spec is not in a good state, with ton
    of open
     >     issues
     >      > without feedback from the editors [4].
     >      >
     >      > So right now I'm at a stage where I think that the feature is
     >     just not
     >      > worth it. It doesn't behave predictably enough for developers,
     >     and you
     >      > have no guarantee of it behaving consistently unless you
    test a
     >      > particular browser, with a particular content in a particular
     >     viewport
     >      > size... That's not great given the current dominant
    position of
     >      > Chromium-based browsers.
     >      >
     >      > On top, issues with scroll anchoring are pretty hard to
     >     unless
     >      > you're aware of the feature.
     >      >
     >      > All in all, it doesn't seem like the kind of feature that
    benefits a
     >      > diverse web (nor web developers for that matter), and I
    think we
     >     should
     >      > remove the feature from Gecko.
     >      >
     >      > Does anyone have strong opinions against removing scroll
     >     anchoring from
     >      > Gecko, based on the above?
     >      >
     >      > Thanks,
     >      >
     >      >   -- Emilio
     >      >
     >      > [1]: https://bugzilla.mozilla.org/show_bug.cgi?id=1519644
     >     <https://bugzilla.mozilla.org/show_bug.cgi?id=1519644
     >      > [2]: https://bugzilla.mozilla.org/show_bug.cgi?id=1561450
     >     <https://bugzilla.mozilla.org/show_bug.cgi?id=1561450
     >      > [3]: https://bugzilla.mozilla.org/show_bug.cgi?id=1584499
     >     <https://bugzilla.mozilla.org/show_bug.cgi?id=1584499
     >      > [4]:
     > https://github.com/w3c/csswg-drafts/labels/css-scroll-anchoring-1
>  <https://github.com/w3c/csswg-drafts/labels/css-scroll-anchoring-1
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