RE: [css3-backgrounds] Should a non-zero border-radius create a new stacking context ?

Well, I don't see a good reason for the current behavior either; so what ? Just as I don't see a good reason - from an authoring standpoint - for opacity changing z-ordering, especially when applied on :hover as it often is. That opacity < 1 does create a new stacking context doesn't only make implementation a lot easier, ease of implementation is the only reason given by CSS3 Color for this behavior. I will grant that applying opacity - or arbitrary transforms - without creating a new stacking context is harder than clipping to the proper geometry but ease of implementation clearly is a reason to make certain features change z-ordering. While the existence of a precedent is not sufficient to prove that this particular feature should do the same, I don't see why implementation considerations should not apply in this case and any number of others. Especially when the CR stage is reached and most browsers fail the clipping requirements as specified.

The argument I think you're making is that this would result in a rather odd overflow discontinuity between border-radius:0 and > 0. But that's no different that what you have between opacity:1 and < 1 or transform:none and transform:scale(1). I honestly don't know why the discontinuity is OK in these cases but not for border-radius. If you've explained why I'm still missing it.

________________________________
From: Brad Kemper [brad.kemper@gmail.com]
Sent: Saturday, November 20, 2010 10:10 AM
To: Sylvain Galineau
Cc: www-style@w3.org
Subject: Re: [css3-backgrounds] Should a non-zero border-radius create a new stacking context ?


On Nov 20, 2010, at 8:25 AM, Sylvain Galineau wrote:

I never said it shouldn't clip to the inner corner curve. That's not at all the point. If the element with border-radius in my original testcase created a new stacking context then the blue box moves below the green one (as it also will if you give the parent opacity < 1, or a transform). It still must clip to the parent's curve within that parent's stacking context.

So no one is questioning whether the element should clip to the border curve.

OK, I thought that was part of what you were questioning, because your statement "It's not  clear to me why this is useful or desirable" seemed to be questioning something that was already in the spec. And the spec has no special implications for stacking contexts that I can see.

The suggestion is that the non-zero radius would result in a different z-order for the blue box.

I see no reason why it would or should.


On Nov 20, 2010, at 8:42 AM, fantasai wrote:

What Brad is saying is that the problem you're running into wrt stacking
contexts has nothing to do with whether the border is curved or square.
(Give the element some negative margins, and you'll get the same behavior
with a square border.)

Having non-zero border radius affect the z-order doesn't follow from the
problem you describe: it's not creating any new or interesting stacking
effects. It's just changing the shape of the clip mask.

Right.