[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.

Author: fantasai

css-overflow-3/Overview.bs

@@ -140,6 +140,36 @@ Scrollable overflow</h3>
 	set of things extending outside of that box's padding edge
 	for which a scrolling mechanism needs to be provided.
 
+	The <dfn>scrollable overflow region</dfn> is the non-rectangular region
+	occupied by the <a>scrollable overflow</a>, and the
+	<dfn>scrollable overflow rectangle</dfn> is
+	the minimal rectangle whose axis is aligned to the box's axes
+	and contains the <a>scrollable overflow region</a>.
+
+	The <a>scrollable overflow region</a> of a box includes:
+	<ul>
+		<li>the box’s own content and padding areas
+		<li>all line boxes directly contained by the box
+		<li>the margin boxes of all in-flow and floated boxes
+		    for which it is the containing block
+		    (accounting for transforms as described below).
+		<li>the padding boxes of all absolutely-positioned boxes
+		    for which it is the containing block
+		    (accounting for transforms as described below).
+
+		    Issue: Blink (and presumably WebKit) use the margin box here. Is that better?
+		<!-- 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 -->
+
+		<li>the <a>scrollable overflow regions</a> of all of the above boxes
+		    (accounting for transforms as described below),
+		    provided they themselves have ''overflow: visible''
+		    (i.e. do not themselves trap the overflow)
+		    and that overflow is not already clipped
+		    by the 'clip' or 'clip-path' properties.
+
+		    Issue: What about masking?
+	</ul>
+
 	<p class="issue">
 		The following definition should be rewritten to use
 		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>
 		the elements should probably count
 		only towards the overflow of the element that flattens them.
 
-	The <dfn>scrollable overflow region</dfn> is the non-rectangular region
-	occupied by the <a>scrollable overflow</a>, and the
-	<dfn>scrollable overflow rectangle</dfn> is
-	the minimal rectangle whose axis is aligned to the box's axes
-	and contains the <a>scrollable overflow region</a>.
-	Note that the <a>scrollable overflow rectangle</a> is a rectangle
-	in the box's coordinate system, but might be non-rectangular
+	Note that the <a>scrollable overflow rectangle</a> is a always rectangle
+	in the box's own coordinate system, but might be non-rectangular
 	in other coordinate systems due to transforms [[CSS3-TRANSFORMS]].
 
 <h3 id="border-box">