Re: [css-transforms] Should transform-style: preserve-3d create containing block for positioned descendants?

On Dec 11, 2014, at 2:24 PM, Robert O'Callahan <robert@ocallahan.org> wrote:
> 
> On Fri, Dec 12, 2014 at 7:47 AM, Simon Fraser <smfr@me.com <mailto:smfr@me.com>> wrote:
> There is disagreement among implementations about whether "transform-style: preserve-3d” creates containing block for positions descendants.
> 
> The working draft[1] says in the prose:
> 
>> A 3D rendering context is established by a a transformable element whose computed value for transform-style is preserve-3d, and which itself is not part of a 3D rendering context. Note that such an element is always a containing block.
> 
> 
> but we don't explicitly call this out under the transform-style property. The new spec text doesn't require that preserve-3d create containing block, and logically I see no reason why it should. (I’m also not sure why perspective has to create a containing block.)
> 
> WebKit/Blink have this behavior because of a bug[2]. Firefox has this behavior too, possibly because they copied WebKit behavior, or went by the spec prose. IE 11 does not have this behavior.
> 
> I think it's simpler if it does. If we don't make it a containing block, then abs-pos children can escape from a preserve-3d parent and it's unclear whether and how the parent's preserve-3d would apply to them.

For both this reason and for compat, it feels like preserve-3d has to create containing block for positioned descendants then. I’m OK with that (modulo web developer surprise when adding causes positioned things to move around).

Simon