[css-overflow] Clarify definition of "immediately preceding"

Ignores intervening absolutely positioned elements and closing element boundaries.

See #10868

Author: frivoal

css-overflow-4/Overview.bs

@@ -714,7 +714,9 @@ Indicating Block-Axis Overflow: the 'block-ellipsis' property</h3>
 
 	This property only affects a line box
 	if it immediately precedes either a [=region break=]
-	or a [=clamp point=] in the [=block formatting context=].
+	or a [=clamp point=] in the [=block formatting context=],
+	ignoring any intervening [=absolutely positioned=] element
+	or any element closing boundary.
 
 	Note: See [[css-overflow-4#fragmentation]] for a way to generate boxes with such a [=region break=].