Add a "cancel" event for when file upload selection is unchanged

Closes #6376.

source

@@ -47916,11 +47916,11 @@ ldh-str = &lt; as defined in <a href="https://tools.ietf.org/html/rfc1034#
  no more than one file in the list of <span data-x="concept-input-type-file-selected">selected
  files</span>.</p>
 
- <p>The element's <span>input activation behavior</span> is to run the following steps:</p>
+ <p>The <span>input activation behavior</span> for such an element <var>element</var> is:</p>
 
  <ol>
- <li><p>If the algorithm is invoked when the element's <code>Window</code> object does not have
- <span>transient activation</span>, then return without doing anything else.</p></li>
+ <li><p>If the algorithm is invoked when <var>element</var>'s <span>relevant global object</span>
+ does not have <span>transient activation</span>, then return.</p></li>
 
  <li>
  <p>Run these steps <span>in parallel</span>:</p>
@@ -47929,15 +47929,33 @@ ldh-str = &lt; as defined in <a href="https://tools.ietf.org/html/rfc1034#
  <li><p>Optionally, wait until any prior execution of this algorithm has terminated.</p></li>
 
  <li><p>Display a prompt to the user requesting that the user specify some files. If the <code
- data-x="attr-input-multiple">multiple</code> attribute is not set, there must be no more than
- one file selected; otherwise, any number may be selected. Files can be from the filesystem or
- created on the fly, e.g., a picture taken from a camera connected to the user's
- device.</p></li>
+ data-x="attr-input-multiple">multiple</code> attribute is not set on <var>element</var>, there
+ must be no more than one file selected; otherwise, any number may be selected. Files can be
+ from the filesystem or created on the fly, e.g., a picture taken from a camera connected to the
+ user's device.</p></li>
 
  <li><p>Wait for the user to have made their selection.</p></li>
 
- <li><p><span>Update the file selection</span> for the <code>input</code> element.</p></li>
+ <li><p>If the user dismissed the prompt without changing their selection, then <span>queue an
+ element task</span> on the <span>user interaction task source</span> given <var>element</var>
+ to <span data-x="concept-event-fire">fire an event</span> named <code
+ data-x="event-cancel">cancel</code> at <var>element</var>, with the <code
+ data-x="dom-Event-bubbles">bubbles</code> attribute initialized to true.</p></li>
+
+ <li><p>Otherwise, <span>update the file selection</span> for <var>element</var>.</p></li>
  </ol>
+
+ <p class="note">As with all user interface specifications, user agents have a good deal of
+ freedom in how they interpret these requirements. The above text implies that a user either
+ dismisses the prompt or changes their selection; exactly one of these will be true. But the
+ mapping of these possibilities to specific user interface elements is not mandated by the
+ standard. For example, a user agent might interpret clicking the "Cancel" button when files were
+ previously selected as a change of selection to select zero files, thus firing <code
+ data-x="event-input">input</code> and <code data-x="event-change">change</code>. Or it might
+ interpret such a click as a dismissal that leaves the selection unchanged, thus firing <code
+ data-x="event-cancel">cancel</code>. Similarly, it's up to the user agent whether re-selecting
+ the same files counts as were previously selected counts as a dismissal, or as a change of
+ selection.</p>
  </li>
  </ol>
 
@@ -123744,8 +123762,8 @@ INSERT INTERFACES HERE
  <tr> <!-- cancel -->
  <td> <dfn event for="HTMLElement"><code data-x="event-cancel">cancel</code></dfn>
  <td> <code>Event</code>
- <td> <code>dialog</code> elements
- <td> Fired at <code>dialog</code> elements when they are canceled by the user (e.g. by pressing the Escape key)
+ <td> <code>dialog</code> elements, <code>input</code> elements
+ <td> Fired at <code>dialog</code> elements when they are canceled by the user (e.g., by pressing the Escape key), or at <code>input</code> elements in the <span data-x="attr-input-type-file">File</span> state when the user does not change their selection
 
  <tr> <!-- change -->
  <td> <dfn event for="HTMLElement"><code data-x="event-change" id="event-input-change">change</code></dfn>