Don't execute scripts that move between documents during fetching

Fixes #2469. Also clarifies surrounding conditions and cleans up the
algorithm by pulling out a variable for the element's node document.

Author: domenic

source

@@ -57958,6 +57958,7 @@ o............A....e
     elements.
    </p>
 
+   <!-- Cleanup? But see https://github.com/whatwg/html/pull/964#issuecomment-204384209 -->
    <li><p>Let <var>settings object</var> be the element's <span>node document</span>'s
    <code>Window</code> object's <span>environment settings object</span>.</p></li>
 
@@ -58221,11 +58222,12 @@ o............A....e
   <p class="note">If a <code>script</code> element that blocks a parser gets moved to another
   <code>Document</code> before it would normally have stopped blocking that parser, it nonetheless
   continues blocking that parser until the condition that causes it to be blocking the parser no
-  longer applies (e.g. if the script is a <span>pending parsing-blocking script</span> because there
-  was <span>a style sheet that is blocking scripts</span> when it was parsed, but then the script is
-  moved to another <code>Document</code> before the style sheet loads, the script still blocks the
-  parser until the style sheets are all loaded, at which time the script executes and the parser is
-  unblocked).</p>
+  longer applies. For example, consider a script that is a <span>pending parsing-blocking
+  script</span> because there was <span>a style sheet that is blocking scripts</span> when it was
+  parsed. If the script is moved to another <code>Document</code> before the style sheet loads, the
+  script still blocks the parser until the style sheets are all loaded, at which time the <span
+  data-x="execute the script block">execute a script block</span> algorithm runs immediately returns
+  without evaluating any script. At this point, the parser is finally unblocked.</p>
 
   <!-- also (and this would be worth testing): the way the spec is written, if you do not have a
   style sheet that is blocking scripts, then you parse a <script src>, then while waiting for the
@@ -58237,46 +58239,52 @@ o............A....e
   block</dfn>, it must run the following steps.</p>
 
   <ol>
+   <li><p>If <span data-x="concept-script-script">the script's script</span> is null, <span
+   data-x="concept-event-fire">fire an event</span> named <code data-x="event-error">error</code>
+   at the element, and return.</p></li>
 
-   <li>
+   <li><p>Let <var>document</var> be the <code>script</code> element's <span>node
+   document</span>.</p></li>
 
-    <p>If the element is flagged as <span>"parser-inserted"</span>, but the element's
-    <span>node document</span> is not the <code>Document</code> of the parser that created the element,
-    then return.</p>
+   <li>
+    <p>If <var>document</var>'s <span>relevant settings object</span> is not equal to <span
+    data-x="concept-script-script">the script's script</span>'s <span>settings object</span>, then
+    return.</p>
 
+    <p class="note">This can occur if, while the script was being fetched inside the <span>prepare a
+    script</span> algorithm, the element was moved to a document in a different
+    <code>Window</code>.</p>
    </li>
 
    <li>
+    <p>If <span data-x="concept-n-noscript">scripting is disabled</span> for the element, then
+    return.</p>
 
-    <p>If <span data-x="concept-script-script">the script's script</span> is null, <span
-    data-x="concept-event-fire">fire an event</span> named <code data-x="event-error">error</code>
-    at the element, and return.</p>
-
+    <p class="note">This can occur if, while the script was being fetched inside the <span>prepare a
+    script</span> algorithm, the element was moved to a document without a <span
+    data-x="concept-document-bc">browsing context</span>.</p>
    </li>
 
-   <!-- SCRIPT EXEC -->
-
    <li>
+    <p>If the element is flagged as <span>"parser-inserted"</span>, but <var>document</var> is
+    not the <code>Document</code> of the parser that created the element, then return.</p>
 
-    <p>If the script is <span data-x="concept-script-external">from an external file</span>, or
-    <span data-x="concept-script-type">the script's type</span> is "<code data-x="">module</code>",
-    then increment the <span>ignore-destructive-writes counter</span> of the <code>script</code>
-    element's <span>node document</span>. Let <var>neutralized doc</var> be that
-    <code>Document</code>.</p>
-
+    <p class="note">This can occur if the element was <span
+    data-x="concept-node-adopt">adopted</span> into a new document during parsing, before the
+    <span>prepare a script</span> algorithm ran.</p>
    </li>
 
-   <li>
+   <!-- SCRIPT EXEC -->
 
-    <p>Let <var>old script element</var> be the value to which the <code>script</code>
-    element's <span>node document</span>'s <code
-    data-x="dom-document-currentScript">currentScript</code> object was most recently
-    set.</p>
+   <li><p>If the script is <span data-x="concept-script-external">from an external file</span>, or
+   <span data-x="concept-script-type">the script's type</span> is "<code data-x="">module</code>",
+   then increment the <span>ignore-destructive-writes counter</span> of
+   <var>document</var>.</p></li>
 
-   </li>
+   <li><p>Let <var>old script element</var> be the value to which <var>document</var>'s <code
+   data-x="dom-document-currentScript">currentScript</code> object was most recently set.</p></li>
 
    <li>
-
     <p>Switch on <span data-x="concept-script-type">the script's type</span>:</p>
 
     <dl class="switch">
@@ -58285,7 +58293,7 @@ o............A....e
       <ol>
        <li>
         <p>If the <code>script</code> element's <span>root</span> is <em>not</em> a <span>shadow
-        root</span>, then set the <code>script</code> element's <span>node document</span>'s <code
+        root</span>, then set <var>document</var>'s <code
         data-x="dom-document-currentScript">currentScript</code> attribute to the
         <code>script</code> element. Otherwise, set it to null.</p>
 
@@ -58311,31 +58319,17 @@ o............A....e
       </ol>
      </dd>
     </dl>
-
-   </li>
-
-   <li>
-
-    <p>Set the <code>script</code> element's <span>node document</span>'s <code
-    data-x="dom-document-currentScript">currentScript</code> attribute to <var>old script
-    element</var>.</p>
-
    </li>
 
-   <li>
-
-    <p>Decrement the <span>ignore-destructive-writes counter</span> of <var>neutralized doc</var>, if it was incremented in the earlier step.</p>
+   <li><p>Set <var>document</var>'s <code data-x="dom-document-currentScript">currentScript</code>
+   attribute to <var>old script element</var>.</p></li>
 
-   </li>
-
-   <li>
-
-    <p>If the script is <span data-x="concept-script-external">from an external file</span>, then
-    <span data-x="concept-event-fire">fire an event</span> named <code
-    data-x="event-load">load</code> at the <code>script</code> element.</p>
-
-   </li>
+   <li><p>Decrement the <span>ignore-destructive-writes counter</span> of <var>document</var>, if it
+   was incremented in the earlier step.</p></li>
 
+   <li><p>If the script is <span data-x="concept-script-external">from an external file</span>, then
+   <span data-x="concept-event-fire">fire an event</span> named <code
+   data-x="event-load">load</code> at the <code>script</code> element.</p></li>
   </ol>
 
   </div>