Use actions queue when dispatching undo actions from "Release Actions"

index.html

@@ -10314,6 +10314,33 @@ <h3><dfn>Release Actions</dfn></h3>
 as if the state was released by an explicit series of actions.
 It also clears all the internal state of the virtual devices.
 
+<p>To <dfn>get undo actions</dfn> given <var>input
+ state</var>:
+
+<ol class=algorithm>
+ <li><p>Let <var>token</var> be a new unique identifier.
+
+ <li><p>Enqueue <var>token</var> in <var>input state</var>&apos;s <a>actions
+  queue</a>.
+
+ <li><p>Wait for <var>token</var> to be the first item
+ in <var>input state</var>&apos;s <a>actions queue</a>.
+
+ <aside class=note>
+   <p>This ensures that only one set of actions can be run at a time,
+   and therefore different actions commands using the same underlying
+   state don&apos;t race. In a session that is only a HTTP session only one
+   command can run at a time, so this will never block. But other
+   session types can allow running multiple commands in parallel, in
+   which case this is necessary to ensure sequential access.
+ </aside>
+
+ <li><p>Let <var>undo actions</var> be <var>input
+ state</var>&apos;s <a>input cancel list</a> in reverse order.
+
+ <li><p>Return <var>undo actions</var>.
+</ol>
+
 <p>The <a>remote end steps</a>, given <var>session</var>, <var>URL
 variables</var> and <var>parameters</var> are:
 
@@ -10323,25 +10350,24 @@ <h3><dfn>Release Actions</dfn></h3>
   code</a> <a>no such window</a>.
 
  <li><p>Let <var>input state</var> be the result of <a>get the
- input state</a> with <a>session</a> and <a>current
- top-level browsing context</a>.
+  input state</a> with <a>session</a> and <a>current
+  top-level browsing context</a>.
 
  <li><p>Let <var>actions options</var> be a new <a>actions options</a>
   with the <a>is element origin</a> steps set to <a>represents a web
   element</a>, and the <a>get element origin</a> steps set
   to <a>get a WebElement origin</a>.
 
- <li><p>Let <var>undo actions</var> be <var>input
- state</var>&apos;s <a>input cancel list</a> in reverse order.
+ <li><p>Let <var>undo actions</var> be the result of <a>get undo actions</a>.
 
- <li><p><a>Try</a> to <a>dispatch tick actions</a> with arguments
-  <var>undo
-  actions</var>, <code>0</code>,<var>session</var>&apos;s <a>current
-  browsing context</a>, and <a>actions options</a>.
+ <li><p><a>Dispatch actions</a> with <var>input state</var>,
+  <var>undo actions</var>, <a>current browsing context</a>,
+  and <var>actions options</var>. If this results in an <a>error</a>
+  return that error.
 
  <li><p><a>Reset the input state</a> with <var>session</var>
- and <var>session</var>&apos;s <a>current top-level browsing
- context</a>.
+  and <var>session</var>&apos;s <a>current top-level browsing
+  context</a>.
 
  <li><p>Return <a>success</a> with data <a><code>null</code></a>.
 </ol>