[css-align] Loosen the abspos default overflow rules to work on anything that's not normal-aligned (per discussion with @fantasai and @bfgeek). Fix references to 'original containing block' to be the bounding rect of the OCB and IMCB, to correctly handle cases where the IMCB is partially or completely outside the OCB.

Author: tabatkins

css-align-3/Overview.bs

@@ -897,33 +897,31 @@ Content Distribution for Scroll Containers</h5>
 <h5 id=auto-safety-position>
 Self-Alignment for Absolutely Positioned Boxes</h5>
 
-	For [=absolutely positioned=] [=alignment subjects=] whose
-
-	* relevant [=self-alignment property=] is not ''justify-self/normal'' and
-	* whose 'inset-area' property successfully defines an [=inset-area grid=],
-		or whose [=self-alignment property=] is ''anchor-center''
-		(aka, the element is "using" Anchor Positioning in some way)
-
+	For [=absolutely positioned=] [=alignment subjects=] 
+	whose relevant [=self-alignment property=] is not ''justify-self/normal'',
 	the default [=overflow alignment=] behavior is as follows:
 
-	1. If the [=alignment subject=] overflows its [=inset-modified containing block=]
+	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 its [=original containing block=]
+		but does not overflow |default overflow rect|
 		align as specified (''unsafe'').
-	2. If the <em>size</em> of the [=alignment subject=]
-		fits within its [=original containing block=],
-		but its specified alignment would cause it to overflow the [=original containing block=],
+	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 [=original containing block=]
+		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).
-	3. If the size of the [=alignment subject=]
-		overflows the [=original containing block=],
-		it is instead start-aligned within the [=original containing block=]
+	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'').
 
-	(For [=absolutely-positioned=] [=alignment subjects=] that fail the above conditions,
+	(For [=absolutely-positioned=] [=alignment subjects=] that fail the above condition,
 	see [[#auto-safety-default]].)
 
 <h5 id=auto-safety-default>