Hi all,

 10/18/19 7:19 PM, Chris Harrelson wrote:

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 should 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 suppress 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 telemetry 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 made 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,

On Thu, Oct 10, 2019 at 2:13 PM Rick Byers <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 normal
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 UA-specific
    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?


On Mon, Oct 7, 2019 at 2:11 PM Mike Taylor <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?


Mike Taylor
Web Compat, Mozilla

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