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Re: [css3-transforms] Effect of CSS transforms on scrollable areas

From: Robert O'Callahan <robert@ocallahan.org>
Date: Thu, 31 May 2012 12:41:07 +1200
Message-ID: <CAOp6jLaKD=vyWXEHoU5Z0T6p3e6xeJVZB97SkBYD3vVz2YqH+A@mail.gmail.com>
To: www-style <www-style@w3.org>, "Tab Atkins Jr." <jackalmage@gmail.com>
Cc: Chris Jones <cjones@mozilla.com>
On Thu, May 31, 2012 at 11:31 AM, Robert O'Callahan <robert@ocallahan.org>wrote:

> In all major browsers, transforms normally affect the scrollable area of a
> scrollable container. However, during an animation or transition, Webkit
> treats the transform as having the value it had the last time it wasn't
> animated/transitioned:
> http://people.mozilla.org/~roc/test_transform_scrollable_area.html
> Presumably this is a performance optimization related to asynchronous
> compositing.
> I think browsers should behave consistently here. Should we spec the
> Webkit behavior? I guess that would be something like
> "Transforms affect the scrollable overflow area as expected, unless
> they're subject to a CSS animation or transition in which case they affect
> the scrollable overflow area as if they had their values from before they
> were subject to an animation/transition."

Actually, Webkit's behavior is more complicated than this. Event
dispatching can (at least in some cases) cause different behavior.

I've updated the testcase. Now if you hover over the animated element while
it's animating, you can observe changes to the result of
getBoundingClientRect during the animation. Curiously, changes to the
scrollbar state are still not reflected until the end of the animation.

I don't think we should spec the details of that mess. However, I don't
know what to do instead.

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Received on Thursday, 31 May 2012 00:41:38 UTC

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