Add <form>'s "firing submission events" flag

Fixes #4620. This flag prevents reentrancy into the submission algorithm
during submit or invalid events. Blink and Gecko implement this for
submit only; WebKit implements it for both. The specification chooses
WebKit's behavior.

Tests: web-platform-tests/wpt#16811

source

@@ -56054,6 +56054,9 @@ fur
   <p>Each <code>form</code> element has a <dfn>constructing entry list</dfn> boolean, initially
   false.</p>
 
+  <p>Each <code>form</code> element has a <dfn>firing submission events</dfn> boolean, initially
+  false.</p>
+
   <p>When a <code>form</code> element <var>form</var> is <dfn
   data-x="concept-form-submit">submitted</dfn> from an element <var>submitter</var>
   (typically a button), optionally with a <var>submitted from <code
@@ -56073,26 +56076,40 @@ fur
    <li><p>Let <var>form browsing context</var> be the <span data-x="concept-document-bc">browsing
    context</span> of <var>form document</var>.</p></li>
 
-   <!-- lock (all the following steps are skipped if called from submit() - see unlock step below) -->
-
-   <li><p>If the <var>submitted from <code data-x="dom-form-submit">submit()</code>
-   method</var> flag is not set, and the <var>submitter</var> element's <span
-   data-x="concept-fs-novalidate">no-validate state</span> is false, then <span>interactively
-   validate the constraints</span> of <var>form</var> and examine the result: if the result
-   is negative (the constraint validation concluded that there were invalid fields and probably
-   informed the user of this) then <span data-x="concept-event-fire">fire an event</span> named
-   <code data-x="event-invalid">invalid</code> at the <var>form</var> element and then return.</p></li>
-
    <li>
     <p>If the <var>submitted from <code data-x="dom-form-submit">submit()</code> method</var> flag
     is not set, then:
 
     <ol>
+     <li><p>If <var>form</var>'s <span>firing submission events</span> is true, then
+     return.</p></li>
+
+     <li><p>Set <var>form</var>'s <span>firing submission events</span> to true.</p></li>
+
+     <li>
+      <p>If the <var>submitter</var> element's <span data-x="concept-fs-novalidate">no-validate
+      state</span> is false, then <span>interactively validate the constraints</span> of
+      <var>form</var> and examine the result. If the result is negative (i.e., the constraint
+      validation concluded that there were invalid fields and probably informed the user of this),
+      then:</p>
+
+      <ol>
+       <li><p><span data-x="concept-event-fire">Fire an event</span> named <code
+       data-x="event-invalid">invalid</code> at the <var>form</var> element.</p></li>
+
+       <li><p>Set <var>form</var>'s <span>firing submission events</span> to false.</p></li>
+
+       <li><p>Return.</p></li>
+      </ol>
+     </li>
+
      <li><p>Let <var>continue</var> be the result of <span data-x="concept-event-fire">firing an
-     event</span> named <code data-x="event-submit">submit</code> at <var>form</var>, with the
-     <code data-x="dom-Event-bubbles">bubbles</code> attribute initialized to true and the <code
+     event</span> named <code data-x="event-submit">submit</code> at <var>form</var>, with the <code
+     data-x="dom-Event-bubbles">bubbles</code> attribute initialized to true and the <code
      data-x="dom-Event-cancelable">cancelable</code> attribute initialized to true.</p></li>
 
+     <li><p>Set <var>form</var>'s <span>firing submission events</span> to false.</p></li>
+
      <li><p>If <var>continue</var> is false, then return.</p></li>
 
      <li>
@@ -56102,11 +56119,7 @@ fur
       data-x="event-submit">submit</code> event could have changed the outcome.</p>
      </li>
     </ol>
-
-   <!-- if you add any steps between the "lock" and "unlock" lines, make sure to update the step
-   immediately before the "lock" line -->
-
-   <!-- unlock -->
+   </li>
 
    <li><p>Let <var>encoding</var> be the result of <span>picking an encoding for the
    form</span>.</p>