[css-position-3] Clarify that fixedpos CB is layout viewport. w3c#6453

Author: fantasai

css-position-3/Overview.bs

@@ -357,7 +357,8 @@ Containing Blocks of Positioned Boxes</h3>
 			the [=fixed positioning containing block=] is:
 
 			* in [=continuous media=],
-				the <a>viewport</a>;
+				the [=layout viewport=]
+				(whose size matches the [=dynamic viewport size=]);
 				as a result, [=fixed=] boxes do not move when the document is scrolled.
 
 				Note: In this respect, they are similar to
@@ -372,7 +373,7 @@ Containing Blocks of Positioned Boxes</h3>
 				as in the case of print preview, for example.)
 
 			Note: As a result, parts of [=fixed-positioned boxes=]
-			that extend outside the [=initial containing block=]/[=page area=]
+			that extend outside the [=layout viewport=]/[=page area=]
 			cannot be scrolled to
 			and will not print.
 	</dl>
@@ -1723,6 +1724,14 @@ 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-3-20230403/">3 April 2023 Working Draft</a>:
+	<ul>
+		<li>Clarified that the [=fixed positioning containing block=]
+			corresponds to the [=layout viewport=] (specifically) in [=continuous media=].
+			(<a href="https://github.com/w3c/csswg-drafts/issues/6453">Issue 6453</a>)
+	</ul>
+
 	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>