[css-position-3] Make top layer boxes use ICB as staticpos rectangle. w3c#9939

Author: fantasai

css-position-3/Overview.bs

@@ -839,6 +839,9 @@ Calculating the Static Position and the “Static-Position Rectangle”</h4>
 	Lastly, all ''margin/auto'' margins on the box itself
 	are treated as zero.
 
+	Boxes in the [=top layer=] always use the [=initial containing block=]
+	as their [=static-position rectangle=].
+
 <h4 id="abspos-breaking" oldids="breaking">
 Fragmenting Absolutely-positioned Elements</h4>
 
@@ -1704,6 +1707,8 @@ Acknowledgments</h2>
 			to, by default, constrain the box to its [=containing block=].
 			See [[#abspos-layout]].
 			(<a href="https://github.com/w3c/csswg-drafts/issues/10316">Issue 10316</a>)
+		<li>Specified the [=static-position rectangle=] of [=top layer boxes=] to be the [=initial containing block=].
+			(<a href="https://github.com/w3c/csswg-drafts/issues/9939">Issue 9939</a>)
 		<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>)