[css-align-3] Allow endward overflow of abspos into scrollable region #12106

Author: fantasai

css-align-3/Overview.bs

@@ -918,6 +918,12 @@ Self-Alignment for Absolutely Positioned Boxes</h5>
 		For this purpose, the |overflow limit rect| is the bounding rectangle
 		of the [=alignment subject's=] [=inset-modified containing block=]
 		and its [=original containing block=].
+		However, because the [=scrollable overflow area=] of a [=scroll container=]
+		can be extended to ensure the visibility of overflowing [=absolutely positioned=] boxes,
+		if the [=original containing block=] is generated by a [=scroll container=]
+		(and is not its [=fixed containing block=]),
+		the |overflow limit rect| is extended to infinity
+		in any direction that does not extend into the [=unreachable scrollable overflow region=].
 	</div>
 
 	(For [=absolutely-positioned=] [=alignment subjects=] that fail the above condition,
@@ -2516,6 +2522,9 @@ Changes</h2>
 
 	Changes since the <a href="https://www.w3.org/TR/2025/WD-css-align-3-20250311/">11 March 2025 Working Draft</a> include:
 	* Clarify that [[#auto-safety-position]] applies when 'position-area' alters the [=used value=] of ''align-self/normal''.
+	* Allow absolutely positioned boxes to honor alignment even when overflowing into
+		the scrollable overflow area of a scroll container containing block.
+		(<a href="https://github.com/w3c/csswg-drafts/issues/12106">Issue 12106</a>)
 
 	See also <a href="https://www.w3.org/TR/2025/WD-css-align-3-20250311/#changes">previous changes</a>.