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Re: Overflow and Margins

From: Ben Cotterell <ben.cotterell@antplc.com>
Date: Mon, 4 Feb 2008 21:24:45 +0000
To: 'CSS Style' <www-style@w3.org>
Message-ID: <1f47b621e8c9d18ea04451762276ee07ce55b575@localhost>

On Mon, Feb 04, 2008 at 10:00:17AM -0800, Brad Kemper wrote:
> 
> On Feb 4, 2008, at 8:48 AM, Ben Cotterell wrote:
[...]
> >><div style="width:400px; overflow:auto">
> >>	<div style="width:800px; margin:50px;">
> >>		<div style="1000px;"></div>
> >>	</div>
> >></div>
> >>
> >>Even in this case, I would prefer to see room for the margin in the
> >>extent box, as though that were a separate viewport. Even if the
> >>current rules don't allow it, I think most designers would expect to
> >>see their specified margins and not understand why they are not
> >>there. Even with all of your patient explanation, I am still having a
> >>difficult time fully grasping why they are not all shown, although I
> >>understand it has something to do with the interpretation of the
> >>rules for over-constrained blocks.
> >
> >The difficulty here is what is the used value of the block's
> >right margin? (This is the 800px wide box I'm talking about).
> >
> >The width available to it is 400px. Its own width is 800px. So it ends
> >up with a used right margin of -450px, not of 50px.
> 
> By my figuring the used right margin would be -400px (the 50px is  
> ignored, and refigured to allow its used width to be equal to its  
> parent), but maybe I am missing something.

Its left margin is also 50px. So you've got:

    50 + 800 + margin-right = 400
    margin-right            = 400 - 800 - 50
    margin-right            = -450

That's how I figure it, but I may have made a mistake somewhere. You get
the idea anyway.

> But the purpose of that huge negative margin is so that it will not  
> impact items outside of the scrollbox. There is no practical reason  
> why that negative margin should similarly impact the size of the  
> extent box. And, it seems to me, it does not impact the size of the  
> extent box.
>
> If it did, the extent box would only be 400px wide. In  
> reality, in my testing, the extent box if big enough to fit even the  
> 1000px object.

Correct, but the point is it's not 50px any more. It's negative and
treated as 0 in this context.

> Since the contents of the extent box are all self contained, the  
> original value of the margin should be used in determining its size.

But which original value? The browser needs a _used value_ to actually
apply and it has to work out that used value with respect to a
containing width.

In this case the computed value is 50px, so it's easy to say the browser
should just use 50px. But what if it's a percentage? Percentage of
what-- the original containing width or the extent box width?

And what if margin-right and margin-left are both auto? How does the
browser decide whether the author wants the border box centered in the
real containing width or in the extent width?

> >Now when the browser comes to compute the extent box for scrolling the
> >outermost div, it's got a -450px gap to work with at the right, not a
> >50px one. It's not obvious how to re-specify this to get the desired
> >50px margin. 10.3.* is all based firmly on the idea that you are  
> >working
> >to some containing width that you know in advance.
> 
> I would specify it as:
> 
> If all of the above have a computed value other than 'auto', the  
> values are said to be "over-constrained" and one of the used values  
> will have to be different from its computed value. If the 'direction'  
> property of the containing block has the value 'ltr', the specified  
> value of 'margin-right' is ignored and the value is calculated so as  
> to make the equality true. If the value of 'direction' is 'rtl', this  
> happens to 'margin-left' instead.
> 
> For the purposes of determining the width of the extent planes within  
> a scrolling box, the equality restriction no longer applies, as the  
> extent box has no restrictions on its width.

If the equality restriction doesn't apply, how do you do centering?

> In this case, the  original value of the margin applies within (and
> only within) that  extent box.

You would also have to explain what the browser is supposed to do if the
original value is a percentage or auto.

> or something like that.

I think the width calculation rules are complicated enough already
without also depending on the container's overflow property.
Received on Monday, 4 February 2008 21:24:57 GMT

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