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Re: [css21] What's the used value of left/right when overconstrained in relpos?

From: Sylvain Galineau <galineau@adobe.com>
Date: Mon, 29 Apr 2013 11:38:41 -0700
To: "Tab Atkins Jr." <jackalmage@gmail.com>, www-style list <www-style@w3.org>
Message-ID: <CDA40951.3233%galineau@adobe.com>

On 4/29/13 11:11 AM, "Tab Atkins Jr." <jackalmage@gmail.com> wrote:

>When doing relpos layout, if both 'left' and 'right' are non-auto, the
>layout is overconstrained.  CSS 2.1 says to ignore the value
>left/right (depending on document direction), and treat it as being
>the negation of the other.
>But what does this actually mean?
>Specifically, CSS 2.1 says this:
>"If neither 'left' nor 'right' is 'auto', the position is
>over-constrained, and one of them has to be ignored. If the
>'direction' property of the containing block is 'ltr', the value of
>'left' wins and 'right' becomes -'left'. If 'direction' of the
>containing block is 'rtl', 'right' wins and 'left' is ignored."
>No mention of used values is made here, so it's unclear exactly what
>it means to "become -'left'", or "[be] ignored".
>We had a long-open bug (from 2009) on this issue in WebKit, and are
>now trying to fix it in Blink
><https://chromiumcodereview.appspot.com/13871003/>.  The engineer
>writing up the patch is assuming that the text above actually means
>that the used value for left/right is the value you solve for.  That
>is, in a situation like this:
>.item {
>  position: relative;
>  left: 100px;
>  right: 20px;
>...the used value for 'right' would be "-100px".  (And that's what
>would be returned by getComputedStyle().right.)
>Note that the spec *does* require this behavior for top/bottom, using
>the proper words.  When overconstrained, the used value of bottom
>explicitly becomes the negation of 'top'.
>However, Chrome, FF, and IE all return "20px" for 'right' in the above
>example. :/  What's the right behavior?
>(Also, the spec is similarly undefined when talking about abspos
>overconstrained situations.  Would be nice to get an answer for that
>as well, hopefully consistent with this one.)

I agree that 'ignored' seems poor wording here; it suggests error

Fwiw the old box model module is rather clear about the constraint fix-up
 applying to the used value in the inline dimensions [1]. As you note,
interop confirms this. I don't think CSS21 or CSSOM ever clearly defined
what getComputedStyle() should return here though.

[1] http://dev.w3.org/csswg/css-box/#blockwidth

Received on Monday, 29 April 2013 18:40:22 UTC

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