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

Author: tabatkins

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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