[css-overflow-3] Use border box, make margin areas effect undefined.

Author: fantasai

css-overflow-3/Overview.bs

@@ -151,12 +151,9 @@ Scrollable overflow</h3>
 		<li>
 			all line boxes directly contained by the box
 		<li>
-			the margin boxes <em>unioned with</em> the border boxes
+			the border boxes
 			of all boxes for which it is the containing block
 			(accounting for transforms as described below).
-			Issue: Only Blink (and presumably WebKit) use the margin box for abspos; other browsers use the border box.
-			Can we convince everyone else to change? Because using the border box is kindof hostile.
-		<!-- 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
@@ -172,6 +169,26 @@ Scrollable overflow</h3>
 			Issue: Need to evaluate compat of honoring or ignoring 'clip' and 'clip-path'.
 	</ul>
 
+	<p>Additionally, the UA <em>may</em> <em>additionally</em> include
+	the margin areas of boxes for which it is the containing block.
+	The conditions under which such margin areas are included is undefined in this level.
+
+	Issue: This needs further testing and investigation; is therefore deferred in this draft.
+	<!--
+	Considerations:
+	  * floats
+	  * relpos
+	  * margins that collapsed through to the edge of the scrollable area
+	  * inline-side margins on both blocks and inlines
+	Notes:
+	  The reasoning for only including border boxes is to minimize the cases
+	  in which overflow scrolling is triggered; however once overflow is triggered,
+	  excluding margins creates tight, uncomfortable edges
+	  because the margin has been effectively truncated.
+	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
+	-->
+
 	<p class="issue">
 		The following definition should be rewritten to use
 		the concept of <a href="https://drafts.csswg.org/css-transforms/#3d-rendering-context">3D rendering context</a> [[!CSS3-TRANSFORMS]]