[css-position-4] Define 'in the same top layer' and 'in a higher top layer' for other specs to reference.

Author: tabatkins

css-position-4/Overview.bs

@@ -363,6 +363,21 @@ This ensures that things like [=popovers=]
 can be displayed reliably,
 regardless of what their ancestor elements might be doing.
 
+The <dfn export>top layer root</dfn> of an element
+is its nearest ancestor element
+that is in the [=Document/top layer=],
+or none otherwise
+(in which case it's painted as part of the document as normal).
+
+Two elements are <dfn export>in the same top layer</dfn>
+if they have the same [=top layer root=]
+(including if both are none).
+An element A is <dfn export>in a higher top layer</dfn>
+than an element B
+if A has a [=top layer root=],
+and either B has a [=top layer root=] earlier in the [=Document/top layer=] than A's,
+or B doesn't have a [=top layer root=] at all.
+
 Note: The [=Document/top layer=] is managed entirely by the [=user agent=];
 it cannot be directly manipulated by authors.
 This ensures that "nested" invocations of top-layer-using APIs,