Re: [css-transforms] 'perspective' establishing stacking context and containing block

On Sat, Feb 22, 2014 at 10:38 PM, Simon Fraser <smfr@me.com> wrote:

> On Feb 22, 2014, at 10:35 am, L. David Baron <dbaron@dbaron.org> wrote:
>
> > http://dev.w3.org/csswg/css-transforms/#perspective-property says:
> >  # The use of this property with any value other than none
> >  # establishes a stacking context. It also establishes a containing
> >  # block (somewhat similar to position: relative), just like the
> >  # transform property does.
> >
> > This doesn't make a whole lot of sense to me.  The normal reason for
> > doing this things is because the effect requires the entire element
> > to be visually atomic (as with opacity, mix-blend-mode, 3-D
> > transforms) or because the element changes coordinate systems (as
> > with all transforms).
> >
> > However, as I understand it, the 'perspective' property doesn't
> > actually do any of these things to the element on which the property
> > is set; it does these things to that element's 3-D transformed
> > children [1] (which in turn already establish stacking contexts and
> > containing blocks because of the transform property).
> >
> > So it seems odd to me that 'perspective' causes these effects.
> >
> >
> > That said, it's what the spec says, and it's what Safari appears to
> > do (at least mobile Safari on iOS 6.1.6), so I'm inclined to change
> > Gecko (which doesn't currently do these things) to match, assuming
> > that other Gecko developers agree with this approach.  I'm curious,
> > though, if others feel the spec should change.
> >
> > Tests are at:
> >  http://dbaron.org/css/test/2014/perspective-stacking-context
> >  http://dbaron.org/css/test/2014/perspective-containing-block-abs
> >  http://dbaron.org/css/test/2014/perspective-containing-block-fixed
> > and also permanently archived at:
> >  http://lists.w3.org/Archives/Public/www-archive/2014Feb/0021.html
> >
> > -David
> >
> > [1] ... or 3-D transformed descendants within the same 3-D rendering
> > context
>
> You're correct in that perspective doesn't strictly-speaking have to create
> stacking context; it simply provides a transformation matrix that is
> multiplied
> into the transforms of descendants that are affected by the perspective.
>
> However, in the OS X and iOS implementations, the underlying CA framework
> applies perspective through the "sublayer transform" property of a Core
> Animation
> layer, and so specifying it this way makes the implementation easier. This
> is particularly
> true when that same framework is being used to run transform animations,
> making it
> not possible to simply do the transform math in the WebKit code on every
> frame.


I agree with David that this does feel odd.

Simon, is this something that we can fix with your new transform-style
property? For instance, we could ignore perspective unless transform-style
is something other than 'auto'.