Inert acts on the flat tree, not shadow-including descendants.

source

@@ -3709,7 +3709,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
     <p>The following terms are defined in the <cite>CSS Scoping</cite>: <ref spec=CSSSCOPING></p>
 
     <ul class="brief">
-     <li><dfn data-x-href="https://drafts.csswg.org/css-scoping/#flat-tree">flat tree</dfn></li>
+     <li><dfn id="flat-tree" data-x-href="https://drafts.csswg.org/css-scoping/#flat-tree">flat tree</dfn></li>
     </ul>
 
     <p>The following terms and features are defined in <cite>CSS Color Adjustment</cite>: <ref
@@ -72478,7 +72478,7 @@ END:VCARD</pre>
      <li><p>Let <var>ineligible</var> be an empty list.</p></li>
      <li>
       <p>While <var>candidate</var> is <span>inert</span>, or <var>candidate</var> is not an an
-      ancestor of <var>originalElement</var>:</p>
+      ancestor of <var>originalElement</var> in the <span>flat tree</span>:</p>
       <ol>
        <li><p>Add <var>candidate</var> to <var>ineligible</var>.</p></li>
        <li><p>Set <var>candidate</var> to the element which would be the <span
@@ -72545,7 +72545,8 @@ END:VCARD</pre>
   made <span>inert</span>.</p>
 
   <p w-nodev>While an element has an <code data-x="attr-inert">inert</code> attribute, the user
-  agent must mark that element and all its shadow-including descendants as <span>inert</span>.</p>
+  agent must mark that element and all its <span>flat tree</span> descendants as
+  <span>inert</span>.</p>
 
   <p class="note">By default, there is no persistent visual indication of a subtree being inert.
   Authors are encouraged to clearly mark what parts of their document are active and which are