Refine description of when CapturedMouseEvent events are dispatched

- Make CaptureController an EventTarget (screen-share#9), temporarily redefining
  it until this is done in the "Screen Capture" spec. Also align the
  way oncapturedmousechange is specified with similar EventHandler
  attributes from the "Media Capture and Streams" spec (i.e. by
  referring to an event type).
- Elaborate in the CapturedMouseEvent section the way events are
  dispatched on CaptureController objects associated to a
  capture-session, defining a new capturedmousechange type for them.
  Also move there the requirements about when to fire the event.

index.html

@@ -76,6 +76,20 @@ <h2>CapturedMouseEvent interface</h2>
         more scrutiny in which information may safely be exposed. At present, only information
         already available to the capturing application is exposed - the mouse coordinates.
       </p>
+      <p>
+        For any <a data-cite="screen-capture/#dfn-capture-session">capture-session</a>
+        that has an associated {{CaptureController}}, events with type
+        <dfn>capturedmousechange</dfn>, which do not bubble, which are not
+        cancelable and which use the {{CapturedMouseEvent}} interface, MUST be
+        regularly dispatched on this {{CaptureController}} to indicate any
+        move of the captured cursor within the [=display surface=] associated
+        to that capture-session, or whether the captured cursor entered/exited
+        that surface.
+      </p>
+      <p>Events MUST NOT be fired more than 30 times per second.</p>
+      <p>
+        Events MUST NOT be fired after the last clone of the original video track is stopped.
+      </p>
       <pre class="idl">
         [Exposed=Window]
         interface CapturedMouseEvent : Event {
@@ -151,26 +165,32 @@ <h2>CapturedMouseEventInit dictionary</h2>
     </section>
     <section id="capture-controller-extensions">
       <h2>CaptureController Extensions</h2>
+      <p>
+        We redefine {{CaptureController}} to enable developers to listen to
+        {{CapturedMouseEvent}} events dispatched by the
+        <a data-cite="screen-capture/#dfn-capture-session">capture-session</a>:
+      </p>
       <pre class="idl">
-        partial interface CaptureController {
+        [Exposed=Window, SecureContext]
+        interface CaptureController : EventTarget {
+          constructor();
+          undefined setFocusBehavior(CaptureStartFocusBehavior focusBehavior);
           attribute EventHandler oncapturedmousechange;
         };
       </pre>
       <dl data-link-for="CaptureController" data-dfn-for="CaptureController">
         <dt>
-          <dfn>oncapturedmousechange</dfn>
+          <dfn>oncapturedmousechange</dfn> of type {{EventHandler}}
         </dt>
         <dd>
-          <p>
-            We extend {{CaptureController}} with {{CaptureController/oncapturedmousechange}}, an
-            {{EventHandler}} for events of type {{CapturedMouseEvent}}.
-          </p>
-          <p>Events MUST NOT be fired more than 30 times per second.</p>
-          <p>
-            Events MUST NOT be fired after the last clone of the original video track is stopped.
-          </p>
+          <p>The event type of this event is {{capturedmousechange}}</p>
         </dd>
       </dl>
+      <div class="issue" data-number="9">
+        Ideally the Screen Capture spec should define {{CaptureController}}
+        as an {{EventTarget}} so that we can just define a partial interface
+        here.
+      </div>
     </section>
   </body>
 </html>