[css-anchor-position] Fix the definitions for position-visibility: anchors-visible

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1008,14 +1008,20 @@ Conditional Hiding: the 'position-visibility' property {#position-visibility}
 
 <pre class=propdef>
 Name: position-visibility
-Value: always | anchors-valid || anchors-visible || no-overflow
+Value: always | [ anchors-valid || anchors-visible || no-overflow ]
 Initial: anchors-visible
 Applies to: [=absolutely-positioned=] elements
 Percentages: n/a
 Inherited: no
 Animation type: discrete
 </pre>
 
+There are times when an element's anchors
+are not appropriate for positioning the element with,
+and it would be better to simply not display the element at all.
+'position-visibility' provides several conditions
+where this could be the case.
+
 <dl dfn-type=value dfn-for=position-visibility>
 	: <dfn>always</dfn>
 	::
@@ -1045,10 +1051,9 @@ Animation type: discrete
 
 	: <dfn>anchors-visible</dfn>
 	::
-		If the element does not have a [=default anchor element=],
-		or it has a [=default anchor element=]
-		but the anchor element is [=clipped by scrolling=],
-		this element is [=strongly hidden=].
+		If the element has a [=default anchor element=]
+		but the anchor element is [=clipped by intervening elements=],
+		this element is also [=strongly hidden=].
 
 		Issue: Since only the [=default anchor element=]
 		triggers scroll-compensation,
@@ -1062,15 +1067,31 @@ Animation type: discrete
 		then the element is [=strongly hidden=].
 </dl>
 
-For the purposes of this property,
-an anchor element is <dfn noexport>clipped by scrolling</dfn>
-if its [=ink overflow rectangle=]
-is fully outside the [=scrollport=]
-of one of its scrollable ancestors.
+<div algorithm>
+	An anchor element |anchor|
+	is <dfn noexport>clipped by intervening elements</dfn>
+	relative to a positioned element |abspos| relying on it
+	if |anchor|'s [=ink overflow rectangle=]
+	is fully clipped by an element
+	which is an ancestor of |anchor|
+	but a descendant of |abspos|'s containing block.
+	This "clipping" can be due to the ancestor being a [=scroll container=],
+	or being clipped to its [=overflow clip edge=]
+	(such as by ''overflow: clip'' or [=paint containment=]).
+
+	Issue: Should any other clipping matter?
+	The 'clip'/'clip-path'/'mask' properties?
+	''opacity: 0''?
+
+	Note: This means that if an abspos is next to its anchor in the DOM,
+	for example,
+	it'll remain visible even if its default anchor is scrolled off,
+	since it's clipped by the same scroller anyway.
 
-Issue: Make sure this definition of clipped
-is consistent with View Transitions,
-which wants a similar concept.
+	Issue: Make sure this definition of clipped
+	is consistent with View Transitions,
+	which wants a similar concept.
+</div>
 
 Making an element <dfn noexport>strongly hidden</dfn>
 makes it act as if it