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

[Emilio Cobos Álvarez]

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
[2]: https://bugzilla.mozilla.org/show_bug.cgi?id=1561450
[3]: https://bugzilla.mozilla.org/show_bug.cgi?id=1584499
[4]: https://github.com/w3c/csswg-drafts/labels/css-scroll-anchoring-1