[cssom-view-1] Clarify offsetParent behavior in fixed position containing blocks (#12531)

Author: zp1996

cssom-view-1/Overview.bs

@@ -1645,14 +1645,16 @@ The <dfn attribute for=HTMLElement>offsetParent</dfn> attribute must return the
     * The element does not have an associated [=CSS/box=].
     * The element is the [=root element=].
     * The element is <a>the <code>body</code> element</a>.
-    * The element's computed value of the 'position' property is ''position/fixed''.
+    * The element's computed value of the 'position' property is ''position/fixed'' and no ancestor establishes a fixed position [=containing block=].
 1. Let <var>ancestor</var> be the parent of the element in the <a>flat tree</a> and repeat these substeps:
-    1. If <var>ancestor</var> is <a>closed-shadow-hidden</a> from the element and its computed value of the 'position' property is ''position/fixed'', terminate this algorithm and return null.
+    1. If <var>ancestor</var> is <a>closed-shadow-hidden</a> from the element, its computed value of the 'position' property is ''position/fixed'', and no ancestor establishes a fixed position [=containing block=], terminate this algorithm and return null.
     1. If <var>ancestor</var> is not <a>closed-shadow-hidden</a> from the element and satisfies at least one of the following, terminate this algorithm and return <var>ancestor</var>.
-        * <var>ancestor</var> is a containing block of absolutely-positioned descendants (regardless of whether there are any absolutely-positioned descendants).
+        * The element is in a fixed position [=containing block=], and <var>ancestor</var> is a containing block for fixed-positioned descendants.
+        * The element is not in a fixed position [=containing block=], and:
+            * <var>ancestor</var> is a containing block of absolutely-positioned descendants (regardless of whether there are any absolutely-positioned descendants).
+            * It is <a>the <code>body</code> element</a>.
+            * The computed value of the 'position' property of the element is ''static'' and the ancestor is one of the following <a>HTML elements</a>: <code>td</code>, <code>th</code>, or <code>table</code>.
         * The element has a different [=effective zoom=] than <var>ancestor</var>.
-        * It is <a>the <code>body</code> element</a>.
-        * The computed value of the 'position' property of the element is ''static'' and the ancestor is one of the following <a>HTML elements</a>: <code>td</code>, <code>th</code>, or <code>table</code>.
     1. If there is no more parent of <var>ancestor</var> in the <a>flat tree</a>, terminate this algorithm and return null.
     1. Let <var>ancestor</var> be the parent of <var>ancestor</var> in the <a>flat tree</a>.