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

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.

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                          https://www.mozilla.org/   𝄂
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