[css-align-3] Adjust abspos smart overflow alignment to handle anchor-center #12020

Author: fantasai

css-align-3/Overview.bs

@@ -778,12 +778,12 @@ Distributed Alignment: the ''<content-distribution>/stretch'', ''space-between''
 <h3 id='overflow-values'>
 Overflow Alignment: the ''safe'' and ''unsafe'' keywords and scroll safety limits</h3>
 
-	When the <a>alignment subject</a> is larger than the <a>alignment container</a>,
-	it will overflow.
-	Some alignment modes, if honored in this situation,
-	may cause data loss:
-	for example, if the contents of a sidebar are centered,
-	when they overflow they may send part of their boxes past the viewport's start edge,
+	In some situations,
+	aligning exactly as specified
+	would cause the [=alignment subject=] to overflow its [=alignment container=],
+	possibly causing data loss.
+	For example, if the contents of a sidebar are unconditionally centered,
+	items large enough to overflow it might extend beyond the viewport's start edge,
 	which can't be scrolled to.
 
 	<div class='example'>
@@ -902,30 +902,31 @@ Self-Alignment for Absolutely Positioned Boxes</h5>
 	the default [=overflow alignment=] behavior is as follows:
 
 	<div algorithm="default overflow self-alignment for abspos">
-		1. Let the |default overflow rect| be the bounding rectangle
-			of the [=alignment subject's=] [=inset-modified containing block=]
-			and its [=original containing block=].
-		2. If the [=alignment subject=] overflows its [=inset-modified containing block=]
-			(its normal [=alignment container=]),
-			but does not overflow |default overflow rect|
-			align as specified (''unsafe'').
-		3. If the <em>size</em> of the [=alignment subject=]
-			fits within the |default overflow rect|,
-			but its specified alignment would cause it to overflow the |default overflow rect|,
-			it is instead shifted the minimum amount
-			to stay within the |default overflow rect|
-			while honoring the alignment as much as possible
-			(i.e., flush with the edge of the [=original containing block=]
-			it was trying to overflow over).
-		4. If the size of the [=alignment subject=]
-			overflows the |default overflow rect|,
-			it is instead start-aligned within the |default overflow rect|
-			(similar to ''safe'').
+		1. If the [=alignment subject=] fits within the [=inset-modified containing block=],
+			align as specified to the extent possible
+			without overflowing the [=inset-modified containing block=].
+		2. Otherwise,
+			if the [=alignment subject=] fits within the |overflow limit rect|,
+			align the [=alignment subject=] such that
+			it fully covers the [=inset-modified containing block=]
+			and is otherwise aligned as specified to the extent possible
+			without overflowing the |overflow limit rect|.
+		3. Otherwise,
+			start-align the [=alignment subject=] within the |overflow limit rect|
+			(similar to ''align-self/safe'').
+
+		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=].
 	</div>
 
 	(For [=absolutely-positioned=] [=alignment subjects=] that fail the above condition,
 	see [[#auto-safety-default]].)
 
+	Note: These rules constrain the position of the box to minimize overflow
+	while honoring the specified alignment insofar as possible,
+	and ensuring continuous behavior as the sizes of the boxes change.
+
 <h5 id=auto-safety-default>
 All Other Alignment</h5>