Store the top-level and parent browsing contexts explicitly.

Previously the parent and top-level browsing contexts were computed
properties based on the current browsing context. This fails in the
case where the frame containing the current browsing context is
removed from the DOM tree since the browsing context is then discarded
meaning that trying to compute its parents will fail. In practice we
expect operations working with the top-level browsing context to
continue working in that case (and indeed implementations are
mostly/entirely not following the current spec), so this makes the
top-level and parent browsing contexts explictly stored properties of
the session that are updated at the point where we change browsing
contexts.

Author: jgraham

index.html

@@ -2160,14 +2160,14 @@ <h2>Sessions</h2>
  representation of a <a>UUID</a>) used to uniquely identify this
  session.  Unless stated otherwise it is <a><code>null</code></a>.
 
-<p>A <a>session</a> has an associated <dfn>current browsing context</dfn>,
- which is the <a>browsing context</a> against which <a>commands</a> will run.
-
-<p>A <a>session</a> has an associated
- <dfn>current top-level browsing context</dfn>, which is the <a>current browsing
- context</a> if it itself is a <a>top-level browsing context</a>, and otherwise
- is the <a>top-level browsing context</a> of the <a>current browsing
- context</a>.
+<p>A <a>session</a> has an associated <dfn>current browsing
+ context</dfn>, which is the <a>browsing context</a> against
+ which <a>commands</a> will run, an associated <dfn>current parent
+ browsing context</dfn>, which is set to the parent of the <a>current
+ browsing context</a> when changing browsing contexts, and an
+ associated <dfn>current top-level browsing context</dfn>, which is
+ set to the top-browsing context ancestor of the <a>current browsing
+ context</a>, when changing browsing contexts.</p>
 
 <p>
 A <a>session</a> has an associated <dfn data-lt="session script timeout|session page load timeout|session implicit wait timeout">session timeouts</dfn>
@@ -2432,10 +2432,9 @@ <h3 id=new-session><dfn data-lt="new sessions|creating a new session">New Sessio
 
  <li><p>Set the <a>webdriver-active flag</a> to true.</p>
 
- <li><p>Set the <a>current top-level browsing context</a>
-  for <var>session</var> in an implementation-specific way. This
-  should be the <a>top-level browsing context</a> of the UA’s
-  <a>current browsing context</a>.
+ <li><p><a>Set the current top-level browsing context</a>
+   for <var>session</var> with the <a>top-level browsing context</a>
+   of the UA’s <a>current browsing context</a>.
 
   <p class="note">WebDriver implementations typically start a
    completely new browser instance, but there is no requirement in
@@ -2932,8 +2931,8 @@ <h3><dfn>Navigate To</dfn></h3>
   <li><p><a>Try</a> to run the <a>post-navigation checks</a>.
  </ol>
 
- <li><p>Set the <a>current browsing context</a>
-  to the <a>current top-level browsing context</a>.
+ <li><p><a>Set the current browsing context</a> with the <a>current
+  top-level browsing context</a>.
 
  <li><p>If the <a>current top-level browsing context</a> contains
   a <a>refresh state pragma directive</a> of <var>time</var> 1 second
@@ -3101,8 +3100,8 @@ <h3><dfn>Refresh</dfn></h3>
    <li><p><a>Try</a> to run the <a>post-navigation checks</a>.
   </ol>
 
- <li><p>Set the <a>current browsing context</a>
-  to the <a>current top-level browsing context</a>.
+ <li><p><a>Set the current browsing context</a> with <a>current
+  top-level browsing context</a>.
 
  <li><p>Return <a>success</a> with data <a><code>null</code></a>.
 </ol>
@@ -3195,6 +3194,28 @@ <h2>Contexts</h2>
   </dl>
 </ol>
 
+<p>When required to <dfn>set the current browsing context</dfn> given
+   a <var>context</var>, an implementation must follow the following
+   steps:
+<ol>
+  <li>Set the <a>current session</a>’s <a>current browsing
+      context</a> to <var>context</var>.
+  <li>Set the <a>current session</a>’s <a>current parent browsing
+    context</a> to the <a>parent browsing context</a>
+    of <var>context</var>, if that context exists, or <a>null</a>
+    otherwise.
+</ol>
+
+<p>When required to <dfn>set the current top-level browsing
+   context</dfn> given a <var>context</var>, an implementation must
+   follow the following steps:
+<ol>
+  <li>Assert: <var>context</var> is a <a>top-level browsing context</a>.
+  <li>Set the <a>current session</a>’s <a>current top-level browsing
+      context</a> to <var>context</var>.
+  <li><a>Set the current browsing context</a> to <var>context</var>.
+</ol>
+
 <p class=note>
 In accordance with
 the <a href=https://html.spec.whatwg.org/multipage/interaction.html#focus>focus</a>
@@ -3292,10 +3313,11 @@ <h3><dfn>Switch To Window</dfn></h3>
   focussing of another <a>top-level browsing context</a>,
   return <a>error</a> with <a>error code</a> <a>unexpected alert open</a>.
 
- <li><p>If <var>handle</var> is equal to the associated <a>window handle</a>
-  for some <a>top-level browsing context</a> in the <a>current session</a>,
-  set the <a>session</a>’s <a>current browsing context</a>
-  to that browsing context.
+ <li><p>If <var>handle</var> is equal to the associated <a>window
+  handle</a> for some <a>top-level browsing context</a> in
+  the <a>current session</a>, let <var>context</var> be the that
+  browsing context, and
+  <a>set the current top-level browsing context</a> with <var>context</var>.
 
   <p>Otherwise, return <a>error</a> with <a>error code</a> <a>no such window</a>.
 
@@ -3455,7 +3477,7 @@ <h3><dfn>Switch To Frame</dfn></h3>
    <dt><var>id</var> is <a><code>null</code></a>
    <dd>
     <ol>
-     <li><p>Set the <a>current browsing context</a> to
+     <li><p><a>Set the current browsing context</a> with
       the <a>current top-level browsing context</a>.
     </ol>
 
@@ -3478,7 +3500,7 @@ <h3><dfn>Switch To Frame</dfn></h3>
       <var>window</var>.<a data-lt='window.[[\getOwnProperty]]'><code>[[\GetOwnProperty]]</code></a>
       (<var>id</var>).
 
-     <li><p>Set the <a>current browsing context</a> to
+     <li><p><a>Set the current browsing context</a> with
       <var>child window</var>’s <a>browsing context</a>.
     </ol>
 
@@ -3496,8 +3518,8 @@ <h3><dfn>Switch To Frame</dfn></h3>
       or <a><code>iframe</code></a> element,
       return <a>error</a> with <a>error code</a> <a>no such frame</a>.
 
-     <li><p>Set the <a>current browsing context</a> to <var>element</var>’s
-      <a>nested browsing context</a>.
+     <li><p><a>Set the current browsing context</a>
+     with <var>element</var>’s <a>nested browsing context</a>.
     </ol>
  </dl>
 
@@ -3539,10 +3561,9 @@ <h3><dfn>Switch To Parent Frame</dfn></h3>
 
  <li><p><a>Handle any user prompts</a> and return its value if it is an <a>error</a>.
 
- <li><p>If the <a>current browsing context</a> is not equal to
-  the <a>current top-level browsing context</a>,
-  set the <a>current browsing context</a> to
-  the <a>parent browsing context</a> of the <a>current browsing context</a>.
+ <li><p>If the <a>current session</a>'s <a>current parent browsing
+ context</a> is not <a>null</a>, <a>set the current browsing
+ context</a> with the <a>current parent browsing context</a>.
 
  <li><p>Update any implementation-specific state that would result
   from the user selecting the <a>current browsing context</a> for