add support for XRVisibilityMaskChangeEvent

index.bs

@@ -1574,6 +1574,7 @@ enum XREye {
 
 [SecureContext, Exposed=Window] interface XRView {
   readonly attribute XREye eye;
+  readonly attribute unsigned long index;
   readonly attribute double? recommendedViewportScale;
 
   undefined requestViewportScale(double? scale);
@@ -1586,6 +1587,8 @@ The {{XRViewGeometry/transform}} is given in it's [=view/reference space=].
 
 The <dfn attribute for="XRView">eye</dfn> attribute describes the [=view/eye=] of the underlying [=view=]. This attribute's primary purpose is to ensure that pre-rendered stereo content can present the correct portion of the content to the correct eye.
 
+The <dfn attribute for="XRView">index</dfn> attribute describes a unique offet of this {{XRView}}. This MUST be used by the experience to identify the {{XRView}} for each {{XREye}}.
+
 The optional <dfn attribute for="XRView">recommendedViewportScale</dfn> attribute contains a UA-recommended viewport scale value that the application can use for a {{XRView/requestViewportScale()}} call to configure dynamic viewport scaling. It is `null` if the system does not implement a heuristic or method for determining a recommended scale. If not null, the value MUST be a numeric value greater than 0.0 and less than or equal to 1.0, and MUST be [=quantization|quantized=] to avoid providing detailed performance or GPU utilization data.
 
 Note: It is suggested to quantize the recommended viewport scale by rounding it to the nearest value from a short list of possible scale values, and using hysteresis to avoid instant changes when close to a boundary value. (This also helps avoid rapidly oscillating scale values which can be visually distracting or uncomfortable.)
@@ -2589,6 +2592,50 @@ The optional <dfn attribute for="XRReferenceSpaceEvent">transform</dfn> attribut
 
 NOTE: situations where {{XRReferenceSpaceEvent/referenceSpace}} or {{XRReferenceSpaceEventInit/referenceSpace}} can be when the headset was doffed and donned between 2 seperate locations. In such cases, if the experience relies on world-locked content, it should warn the user and reset the scene.
 
+XRVisibilityMaskChangeEvent {#xrvisibilitymaskchangeevent-interface}
+--------------
+
+Because the frustum does not intersect precisely with the rectangular display, the entire area of the {{XRLayer}} MAY not be displayed. This event will inform the experience about the region of the {{XRView}} that is displayed to the user.
+
+The {{XRVisibilityMaskChangeEvent}} event is fired when the user agent wants to inform the experience that the displayed area of the {{XRLayer}} changed. The experience MAY choose to only draw that region which MAY help with performance. 
+
+NOTE: the experience MUST register this event during the promise resolution of {{XRSystem/requestSession}}. Otherwise, the event may fire and the mask will be lost to the experience.
+
+<pre class="idl">
+[SecureContext, Exposed=Window]
+interface XRVisibilityMaskChangeEvent : Event {
+  constructor(DOMString type, XRVisibilityMaskChangeEventInit eventInitDict);
+  [SameObject] readonly attribute XRSession session;
+  readonly attribute XREye eye;
+  readonly attribute unsigned long index;
+  [SameObject] readonly attribute Float32Array vertices;
+  [SameObject] readonly attribute Uint32Array indices;
+};
+
+dictionary XRVisibilityMaskChangeEventInit : EventInit {
+  required XRSession session;
+  required XREye eye;
+  required Float32Array vertices;
+  required Uint32Array indices;
+};
+</pre>
+
+The <dfn attribute for="XRVisibilityMaskChangeEvent">session</dfn> attribute indicates the {{XRSession}} that generated the event.
+
+The <dfn attribute for="XRVisibilityMaskChangeEvent">eye</dfn> attribute indicates which {{XREye}} the mask applies to.
+
+The <dfn attribute for="XRVisibilityMaskChangeEvent">index</dfn> attribute indicates which {{XRView/index}} the mask applies to.
+
+The <dfn attribute for="XRVisibilityMaskChangeEvent">vertices</dfn> attribute is a [=/list=] of <code>X</code>, <code>Y</code> coordinates. The experience MUST assume that the <code>Z</code> coordinate is <code>-1</code>. Each <code>X</code>, <code>Y</code>, <code>Z</code> coordinate describes a vertex. 
+If this array is empty, the whole region of the {{XRView}} SHOULD be drawn.
+
+The <dfn attribute for="XRVisibilityMaskChangeEvent">indices</dfn> attribute is a [=/list=] of indices that describe the index into the vertex list described by {{XRVisibilityMaskChangeEvent/vertices}}. These indices will describe the region of the eye's {{XRView}} that SHOULD be drawn.
+If this array is empty, the whole region of the {{XRView}} SHOULD be drawn.
+
+The area MUST be drawn using the {{XRViewGeometry/projectionMatrix}} of the {{XRView}} of {{XRVisibilityMaskChangeEvent/eye}} and a default {{XRRigidTransform}}.
+
+NOTE: this means that the area MUST NOT use the {{XRRigidTransform}} of the current {{XRView}} of {{XRVisibilityMaskChangeEvent/eye}}.
+
 Event Types {#event-types}
 -----------