[css-position-3] Update CB definition for fragmented inlines w3c#8284

Author: fantasai

css-position-3/Overview.bs

@@ -317,22 +317,21 @@ Containing Blocks of Positioned Boxes</h3>
 					(for example, see [[CSS-GRID-1#abspos-items]]).
 
 				<dt>If the ancestor is an [=inline box=],
+					using the [=writing mode=] of that box,
 				<dd>
-					the [=containing block=] is formed by
-					the [=block-start=] and [=inline-start=] [=content edges=] of the first [=box fragment=] of the ancestor,
-					and the [=block-end=] and [=inline-end=] [=content edges=] of the last [=box fragment=] of the ancestor.
+					the [=containing block=] is formed by forming a rectangle from
+					the [=start=]-most [=content edges=] (in both axes) of the first [=box fragment=] of the ancestor,
+					and the [=end=]-most [=content edges=] of the [=end=]-most [=box fragment=](s) of the ancestor in each axis.
 					If there are multiple fragments on the same line
 					(e.g. due to [[css-writing-modes-3#bidi-box-model|bidi reordering]]),
-					take the [=start=]most/[=end=]most fragment
-					as the first/last fragment.
+					take the [=start=]-most fragment
+					as the first fragment.
 
-					However, since a [=containing block=] cannot have negative area:
-					if the [=block-end=] edge would be [=start=]wards of the [=block-start=] edge,
-					then the [=block-end=] edge of the [=containing block=]
-					instead uses the [=block-start=] edge;
-					and analogously for the [=inline axis=].
-					(Thus the [=containing block=] will have zero height and/or width as a result.)
+					ISSUE(8284): What is a useful containing block to form
+					when the box is fragmented across multiple lines?
 
+					Note: The [=containing block=] formed by a fragmented [=inline box=]
+					was undefined in [[CSS2]].
 			</dl>
 
 			If no ancestor establishes one,
@@ -1719,6 +1718,15 @@ Acknowledgments</h2>
 
 <h2 class="no-num" id="changes">Changes</h2>
 
+	The following significant changes were made since the
+	<a href="https://www.w3.org/TR/2023/WD-css-position-3-20230217/">17 February 2023 Working Draft</a>:
+	<ul>
+		<li>Updated the [[#def-cb|containing block formed by inline boxes fragmented across multiple lines]]
+			to use the endmost edge of all the fragments in each axis;
+			this proposed definition is still under discussion, however.
+			(<a href="https://github.com/w3c/csswg-drafts/issues/8284">Issue 8284</a>)
+	</ul>
+
 	The following significant changes were made since the
 	<a href="https://www.w3.org/TR/2022/WD-css-position-3-20220901/">01 September 2022 Working Draft</a>:
 	<ul>