Re: Utility of background-composite and background-blend-mode?

On Thu, Mar 7, 2013 at 1:24 PM, Rik Cabanier <cabanier@gmail.com> wrote:

> On Wed, Mar 6, 2013 at 3:25 PM, Robert O'Callahan <robert@ocallahan.org>wrote:
>
>> I gave you an example up above: "An ancestor that created a stacking
>> context just because it's absolute-positioned with z-index not 'auto' ".
>>
>
> that one actually creates a buffer too.
> Here is a test file that I created to verify the behavior of stacking
> contexts on background blending:
>
>
> https://github.com/WebKit/webkit/blob/master/LayoutTests/css3/compositing/effect-background-blend-mode-stacking.html
>
>
> That file doesn't use z-index but I tried that locally. It's the same with
> or without z-index and it renders the same on WK and FF.
>

I assure you that positioned elements with z-index are not always allocated
buffers in Gecko.

You *may* get buffers in some cases. A single fixed-pos child of the
viewport, with or without a z-index, will probably get its own buffer so we
can scroll efficiently. Maybe that's why you saw a buffer in your testcase.

As David said in his post, browsers do a lot of complex optimizations here.
That means we'll need a large test suite for these new blending features.
It also means we don't want the spec to forbid those optimizations or force
us to add to their complexity.

Just to give you more of a taste of what Gecko does:
-- a static abs-pos child of the viewport would not normally get a buffer
-- but if there's transformed or animated content *under* it, it might
-- content in a scrollable div will initially be placed into the same
buffer as the rest of the page
-- until you start scrolling it, at which point it gets its own buffer, and
any content overlaying it will get its own buffer too
-- when you stop scrolling, it will keep its own buffer for a while and
then dynamically switch back to being part of the page
The last three points apply even though the entire page is in a single
stacking context.

Rob
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