On 9/27/19 3:02 PM, Kenji Baheux wrote:
Are these compat issues specific to Firefox, or do they also trigger
weird behaviors on Chrome? Do you have a sense of the size and
convergence for the problematic cases?
Some of the ones we've seen reported are specific to Firefox in the
sense that they only happen in Firefox because we block some tracking
code on the page, for example.
But test-cases without that difference like [1] do happen on Chrome as
well. Fixing that in turn causes other interop issues.
Other sites for which I've investigated scroll anchoring issues just
eternally burn CPU (this is fixed in Firefox, but still happens in
Chrome). STR for example:
* Go to http://lembarsaham.com/
* Scroll to the middle of the page.
* On the devtools console, do: window.addEventListener("scroll", () =>
console.log("scroll"))
* In Chrome, you'll see scroll event listeners being fired constantly,
each of those trashing layout to do literally nothing and burning CPU.
I don't think the size of the compat issues is spectacularly big, it's
just somewhat frustrating because fixing some of them causes others to
break, and is just a very thin line to walk.
If heuristics between Chrome and Firefox differ, sites that only test on
Chrome (specially on mobile) will break on Firefox, and the failure mode
is terrible.
If the heuristics are the same, then Firefox still gets compat issues
for those sites, because of stuff like tracking protection.
-- Emilio
[1]: https://bugzilla.mozilla.org/attachment.cgi?id=9087497
On Fri, Sep 27, 2019, 21:23 Emilio Cobos Álvarez <emi...@mozilla.com
<mailto:emi...@mozilla.com>> 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 implement 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 Gecko, but
I've
done a ton of work over the last few months to fix compat issues 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 reproduces 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], reported
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 diagnose 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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