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Re: [css-will-change] establishing containing block for fixed-positioned elements

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Fri, 4 Dec 2015 16:07:25 -0800
Message-ID: <CAAWBYDBvcnVgaWBZPxVx2aKEuHZt2+HtfRUP3HGj+7z4Td5T9g@mail.gmail.com>
To: "L. David Baron" <dbaron@dbaron.org>
Cc: www-style list <www-style@w3.org>
On Fri, Dec 4, 2015 at 3:11 PM, L. David Baron <dbaron@dbaron.org> wrote:
> On Friday 2015-12-04 14:31 -0800, Tab Atkins Jr. wrote:
>> On Tue, Nov 24, 2015 at 4:28 PM, L. David Baron <dbaron@dbaron.org> wrote:
>> > https://drafts.csswg.org/css-will-change/#valdef-will-change-custom-ident
>> > says:
>> >
>> >   If any non-initial value of a property would cause the element to
>> >   generate a containing block for fixed-position elements,
>> >   specifying that property in will-change must cause the element to
>> >   generate a containing block for fixed-position elements.
>> >
>> > I think this should instead say:
>> >
>> >   If any non-initial value of a property would cause the element to
>> >   generate a containing block for fixed-position elements,
>> >   specifying that property in will-change must cause the element to
>> >   generate a containing block for fixed-position _and
>> >   absolute-position_ elements.
>> >
>> > I don't think we need special will-change handling for the
>> > properties that establish a containing block for absolute-position
>> > but not fixed-position elements (i.e., the position property), but
>> > the properties that establish a containing block for
>> > fixed-positioned elements *also* do so for absolutely-positioned
>> > elements.  And it would be bizarre (and defeat the point of the
>> > special will-change handling) to establish only half of the
>> > containing-block nature and not all of it.
>> >
>> > (Also see
>> > https://lists.w3.org/Archives/Public/public-fx/2015OctDec/0035.html
>> > about making this clearer in css-transforms and css-filters.)
>> >
>> >
>> > While here, it's probably also worth using "absolutely positioned"
>> > and "fixed positioned" as
>> > https://drafts.csswg.org/css-containment/#containment-paint does,
>> > rather than "fixed-position" and "absolute-position".
>>
>> Fixed.
>
> Hmm.  The way you fixed this is extra work that I'm not sure is
> needed, although maybe it's better future-proofing that way.  In
> particular, this requires a separate condition for 'will-change:
> position', since the 'position' property can cause creation of a
> containing block for absolutely positioned elements but not for
> fixed positioned elements.
>
> In other words, you took the "I don't think we need" branch of my
> message above.
>
> But if you prefer it that way, I'd be ok with it.

Why do you think that abspos-CB generators don't need to be handled?

~TJ
Received on Saturday, 5 December 2015 00:08:13 UTC

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