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Re: Proposal for "overflow:clip" for stronger painting isolation

From: Dirk Schulze <dschulze@adobe.com>
Date: Fri, 20 Sep 2013 13:02:01 -0700
To: "intelligentdesigner@timgallantcreative.com" <intelligentdesigner@timgallantcreative.com>
CC: "Tab Atkins Jr." <jackalmage@gmail.com>, www-style list <www-style@w3.org>
Message-ID: <B65A7945-1680-4396-8D62-C327251AA6DD@adobe.com>

On Sep 20, 2013, at 9:40 PM, intelligentdesigner@timgallantcreative.com wrote:

> Could you clarify what this accomplishes beyond the existing CSS clip 
> property?

clip does just work with absolute positioned elements. You may can use clip-path and implementations could optimize for that. But a new keyword for overflow might be adapted a lot easier by web developers (and is less code and less complexity for authors and UAs).


> Tim Gallant
> -----Original Message----- 
> From: Tab Atkins Jr.
> Sent: Friday, September 20, 2013 2:03 PM
> To: www-style list
> Subject: Fwd: Proposal for "overflow:clip" for stronger painting isolation
> I've been talking with out layout/compositor people here on Blink for
> a while now, and one of the things that has shaken out as a big helper
> in getting things to work *fast* is having strong, guaranteed methods
> to isolate parts of the page.
> In particular, isolating the painting of an element seems like a big
> win - having some way to guarantee that the element can be
> hard-clipped to its boundary without losing anything important.
> overflow:hidden doesn't *quite* do this - abspos and fixpos elements
> can escape the bounds of the element.
> As well, overflow:hidden still allows scrolling into the hidden area,
> through JS.  This isn't an edge-case - this technique is used by a lot
> of JS libraries.  This means that we can't depend on the element only
> painting its visible area - we have to speculatively paint further, so
> we can scroll smoothly at a moment's notice.
> To get around both of these issues and provide stronger optimization
> guarantees to the browser, I suggest we add a "clip" or "contained"
> value to overflow.  This does a strong clipping - nothing inside the
> element can paint outside the element's rect.
> It also acts as a positioning root for abspos inside the element, and
> kills fixpos, reverting it to abspos with the element as its
> positioning root.
> The value also completely prevents scrolling - the scrollWidth/Height
> of the element is its visible width/height.
> Thoughts?
> ~TJ 
Received on Friday, 20 September 2013 20:02:28 UTC

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