[css-anchor-positioning-1] Add position-fallback-bounds to address #8724

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1090,6 +1090,73 @@ This limit must be <em>at least</em> five.
 	</pre>
 </div>
 
+Applying Stronger Fallback Bounds: the 'position-fallback-bounds' property {#fallback-bounds}
+--------------------------------------------------------------------------
+
+When an element using [=anchor positioning=]
+is using ''position: absolute'',
+it determines whether or not it's overflowing
+(and thus should try a different fallback position)
+by looking at its (scroll-adjusted) [=inset-modified containing block=].
+By carefully selecting where in the DOM
+the positioned element lives,
+and what element establishes its containing block,
+you can choose a useful element
+to use for its overflow bounds.
+
+When using ''position: fixed'',
+or things like the Popover API
+that use the [=top layer=],
+you lose this ability;
+their containing block is always the viewport
+or the root element's containing block.
+The 'position-fallback-bounds' property
+restores this ability,
+allowing an element to explicitly select
+what element it wants to use
+for checking overflow against.
+
+<pre class=propdef>
+Name: position-fallback-bounds
+Value: normal | <<dashed-ident>>
+Initial: normal
+Applies to: [=absolutely positioned=] elements
+Inherited: no
+Animation type: discrete
+</pre>
+
+<dl dfn-type=value dfn-for=position-fallback-bounds>
+	: <dfn>normal</dfn>
+	:: The element uses its normal (scroll-adjusted, inset-modified) containing block
+		to determine if it's overflowing
+		for the purpose of selecting a [=position fallback list=] entry.
+
+	: <<dashed-ident>>
+	::
+		Let |target| be a [=target anchor element=],
+		given this element
+		and the <<dashed-ident>>.
+
+		If |target| doesn't exist,
+		this behaves as ''position-fallback-bounds/normal''.
+
+		If |target| exists,
+		treat |target| as if it establishes an [=automatic positioning containing block=].
+		Let |target rect| be that containing block,
+		scroll-adjusted if necessary.
+
+		When [=determining the position fallback styles=] of the element,
+		compare it to <em>both</em> its normal containing block
+		and to |target rect|,
+		and treat it as overflowing if it overflows <em>either</em> rectangle.
+</dl>
+
+Issue: I need an editorial rewrite of the scroll-adjusted concept
+so I can properly invoke it here.
+
+
+
+
 Fallback and Automatic Positioning {#fallback-automatic}
 ----------------------------------