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[cssom-view] Add a "smooth" parameter to scrollTo and scrollBy functions

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Tue, 19 Mar 2013 10:46:03 -0700
Message-ID: <CAAWBYDDS7Y4_Tatre0M9Cj+=rbL6ozESOWSFK9y7Pj49F70=HA@mail.gmail.com>
To: www-style list <www-style@w3.org>
Cc: Nat Duca <nduca@google.com>, James Simonsen <simonjam@chromium.org>, Tony Gentilcore <tonyg@google.com>, Tom Wiltzius <wiltzius@google.com>
I'm forwarding a request from some of our internal engineers here, but
I think it's a good idea myself. ^_^

Apparently, there has been a significant rise in the number of sites
desiring "smooth" scrolling when adjusting the scroll position of a
page.  Right now, they're limited to some fairly hacky, bad solutions,
akin to standard Javascript transitions - run a requestAnimationFrame
and update scrollTop in increments.

This is problematic for several reasons.  For one, it's janky, slow,
and power-intensive, just like all javascript-controlled animations.
Moving a lot of these types of transitions/animations to CSS has been
a huge win on several fronts, and it would be nice to extend this same
win to scrolling.

For two, it's difficult to get the subtleties of the behavior down
correctly.  If somebody uses their mousewheel during the transition,
for example, you want to stop the transition immediately and scroll
from that point as appropriate.  Expecting authors to get this kind of
behavior right every time is unrealistic, and if they don't, users
have a bad experience.

For three, it requires a layout, even when you're just doing a
scrollBy to move the page downward by 10px, because you have to
measure offsetTop.

Because of this, I believe we should address the problem directly, and
add a method to say that a given instant-scroll should be "smooth"
rather than instant.

There are a few ways we could do this.  One early idea was to make a
CSS property, like scroll-transition, that controlled this.  I don't
think this is a good idea - I can see good reasons to want both
instant and smooth scrolling on the same element in different
circumstances, which argues that the decision should be made at the
scroll site, not the element.

Thus, I propose that we amend the existing scrollTo and scrollBy
functions in CSSOM View to take a third parameter: an optional
"smooth" string.  If omitted, the scroll is instant.

This achieves the desired behavior at a minimal cost.  In downlevel
clients, the scroll would still work, but would be instant instead of
smooth, which is the same desirable failure mode that transitions have
today.

Thoughts?

~TJ
Received on Tuesday, 19 March 2013 17:46:51 GMT

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