[webkit-dev] [blink-dev] Re: What to do about scroll anchoring?

Emilio Cobos Álvarez emilio at mozilla.com
Tue Oct 29 15:03:34 PDT 2019

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 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,
> Chris
> On Thu, Oct 10, 2019 at 2:13 PM Rick Byers <rbyers at 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?
>> Rick
>> On Mon, Oct 7, 2019 at 2:11 PM Mike Taylor <miket at 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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