Fixing issue #4198: VR deep-linking / VR navigation doesn't work. (#4199)

* Fixing issue #4198: VR deep-linking / VR navigation doesn't work.

* Addressing feedback from Diego.

* One more feedback from Diego.

Author: Artyom17

src/core/scene/a-scene.js

@@ -1,4 +1,4 @@
-/* global Promise, screen */
+/* global Promise, screen, CustomEvent */
 var initMetaTags = require('./metaTags').inject;
 var initWakelock = require('./wakelock');
 var loadingScreen = require('./loadingScreen');
@@ -309,6 +309,14 @@ module.exports.AScene = registerElement('a-scene', {
 
         // Callback that happens on enter VR success or enter fullscreen (any API).
         function enterVRSuccess () {
+          // vrdisplaypresentchange fires only once when the first requestPresent is completed;
+          // the first requestPresent could be called from ondisplayactivate and there is no way
+          // to setup everything from there. Thus, we need to emulate another vrdisplaypresentchange
+          // for the actual requestPresent. Need to make sure there are no issues with firing the
+          // vrdisplaypresentchange multiple times.
+          var event = new CustomEvent('vrdisplaypresentchange', {detail: {display: utils.device.getVRDisplay()}});
+          window.dispatchEvent(event);
+
           self.addState('vr-mode');
           self.emit('enter-vr', {target: self});
           // Lock to landscape orientation on mobile.

src/utils/device.js

@@ -9,6 +9,9 @@ window.addEventListener('vrdisplayactivate', function (evt) {
   if (navigator.xr) { return; }
   canvasEl = document.createElement('canvas');
   vrDisplay = evt.display;
+  // We need to make sure the canvas has a WebGL context associated with it.
+  // Otherwise, the requestPresent could be denied.
+  canvasEl.getContext('webgl', {});
   // Request present immediately. a-scene will be allowed to enter VR without user gesture.
   vrDisplay.requestPresent([{source: canvasEl}]).then(function () {}, function () {});
 });