[css-anchor-position] First draft of position-visibility

css-anchor-position-1/Overview.bs

@@ -1003,6 +1003,82 @@ the ''anchor()'' function resolves to its specified fallback value.
 If no fallback value is specified,
 it resolves to ''0px''.
 
+Conditional Hiding: the 'position-visibility' property {#position-visibility}
+------------------------------------------------------
+
+<pre class=propdef>
+Name: position-visibility
+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>
+
+<dl dfn-type=value dfn-for=position-visibility>
+ : <dfn>always</dfn>
+ ::
+ This property has no effect.
+ (The element is displayed without regard for its anchors
+ or its overflowing status.)
+
+ : <dfn>anchors-valid</dfn>
+ ::
+ If any of the element's [=required anchor references=]
+ do not resolve to a [=target anchor element=],
+ the element is [=strongly hidden=].
+
+ Issue: Does misused anchor() count, like ''border-width: anchor(--foo)''?
+ I assume no, so we'll need to split out the concept of invalid usage
+ from a valid usage that fails to find an anchor.
+
+ Issue: What is a <dfn dfn for>required anchor reference</dfn>?
+ ''anchor()'' functions that don't have a fallback value;
+ the default anchor *sometimes*?
+ Need more detail here.
+
+ Issue: <em>Any</em> anchors are missing,
+ or <em>all</em> anchors are missing?
+ I can see use-cases for either, potentially.
+ Do we want to make a decision here, or make it controllable somehow?
+
+ : <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=].
+
+ Issue: Since only the [=default anchor element=]
+ triggers scroll-compensation,
+ it's really the only one that makes sense to condition this on.
+ Right?
+
+ : <dfn>no-overflow</dfn>
+ ::
+ If the element overflows its [=inset-modified containing block=],
+ even after applying 'position-try',
+ 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.
+
+Issue: Make sure this definition of clipped
+is consistent with View Transitions,
+which wants a similar concept.
+
+Making an element <dfn noexport>strongly hidden</dfn>
+makes it act as if it
+and all of its descendants
+were ''visibility: hidden'',
+regardless of what their actual 'visibility' value is.
+
+
 
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