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[css-overflow-3] Define scrollable overflow for non-transformed stuff more completely. Don't integrate much with the transforms stuff, because I have no idea what any of that says.
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css-overflow-3/Overview.bs

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@@ -140,6 +140,36 @@ Scrollable overflow</h3>
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set of things extending outside of that box's padding edge
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for which a scrolling mechanism needs to be provided.
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The <dfn>scrollable overflow region</dfn> is the non-rectangular region
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occupied by the <a>scrollable overflow</a>, and the
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<dfn>scrollable overflow rectangle</dfn> is
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the minimal rectangle whose axis is aligned to the box's axes
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and contains the <a>scrollable overflow region</a>.
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The <a>scrollable overflow region</a> of a box includes:
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<ul>
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<li>the box’s own content and padding areas
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<li>all line boxes directly contained by the box
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<li>the margin boxes of all in-flow and floated boxes
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for which it is the containing block
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(accounting for transforms as described below).
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<li>the padding boxes of all absolutely-positioned boxes
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for which it is the containing block
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(accounting for transforms as described below).
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Issue: Blink (and presumably WebKit) use the margin box here. Is that better?
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<!-- testcase: http://software.hixie.ch/utilities/js/live-dom-viewer/?%3C!DOCTYPE%20html%3E%0A%3Cstyle%3E%0A%20%20div%20%7B%20height%3A%2050px%3B%20overflow%3A%20scroll%3B%20border%3A%20solid%3B%20position%3A%20relative%3B%7D%0A%20%20p%20%7B%20margin%3A%2025px%3B%20background%3A%20red%3B%20%7D%0A%20%20q%20%7B%20position%3A%20absolute%3B%20background%3A%20blue%3B%20top%3A%2025px%3B%20margin%3A%2025px%3B%20%7D%0A%3C%2Fstyle%3E%0A%3Cdiv%3E%0A%20%20%3Cp%3ETest%20%3Cq%3ETest2%0A%3C%2Fdiv%3E -->
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<li>the <a>scrollable overflow regions</a> of all of the above boxes
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(accounting for transforms as described below),
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provided they themselves have ''overflow: visible''
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(i.e. do not themselves trap the overflow)
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and that overflow is not already clipped
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by the 'clip' or 'clip-path' properties.
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Issue: What about masking?
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</ul>
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<p class="issue">
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The following definition should be rewritten to use
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the concept of <a href="http://dev.w3.org/csswg/css-transforms/#3d-rendering-context">3D rendering context</a> [[!CSS3-TRANSFORMS]]
@@ -201,13 +231,8 @@ Scrollable overflow</h3>
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the elements should probably count
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only towards the overflow of the element that flattens them.
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The <dfn>scrollable overflow region</dfn> is the non-rectangular region
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occupied by the <a>scrollable overflow</a>, and the
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<dfn>scrollable overflow rectangle</dfn> is
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the minimal rectangle whose axis is aligned to the box's axes
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and contains the <a>scrollable overflow region</a>.
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Note that the <a>scrollable overflow rectangle</a> is a rectangle
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in the box's coordinate system, but might be non-rectangular
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Note that the <a>scrollable overflow rectangle</a> is a always rectangle
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in the box's own coordinate system, but might be non-rectangular
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in other coordinate systems due to transforms [[CSS3-TRANSFORMS]].
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<h3 id="border-box">

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