feat(conference): restore video after screen sharing

Will restore the camera video state from before screen sharing was
started (and will mute it if it was muted).

conference.js

@@ -1111,6 +1111,71 @@ export default {
 
     videoSwitchInProgress: false,
 
+    /**
+     * This fields stores a handler which will create a Promise which turns off
+     * the screen sharing and restores the previous video state (was there
+     * any video, before switching to screen sharing ? was it muted ?).
+     *
+     * Once called this fields is cleared to <tt>null</tt>.
+     * @type {Function|null}
+     */
+    _untoggleScreenSharing: null,
+
+    /**
+     * Creates a Promise which turns off the screen sharing and restores
+     * the previous state described by the arguments.
+     *
+     * This method is bound to the appropriate values, after switching to screen
+     * sharing and stored in {@link _untoggleScreenSharing}.
+     *
+     * @param {boolean} didHaveVideo indicates if there was a camera video being
+     * used, before switching to screen sharing.
+     * @param {boolean} wasVideoMuted indicates if the video was muted, before
+     * switching to screen sharing.
+     * @return {Promise} resolved after the screen sharing is turned off, or
+     * rejected with some error (no idea what kind of error, possible GUM error)
+     * in case it fails.
+     * @private
+     */
+    _turnScreenSharingOff(didHaveVideo, wasVideoMuted) {
+        this._untoggleScreenSharing = null;
+        this.videoSwitchInProgress = true;
+        APP.remoteControl.receiver.stop();
+        let promise = null;
+
+        if (didHaveVideo) {
+            promise = createLocalTracks({ devices: ['video'] })
+                .then(([stream]) => this.useVideoStream(stream))
+                .then(() => {
+                    JitsiMeetJS.analytics.sendEvent(
+                        'conference.sharingDesktop.stop');
+                    logger.log('switched back to local video');
+                    if (!localVideo && wasVideoMuted) {
+                        return Promise.reject('No local video to be muted!');
+                    } else if (wasVideoMuted && localVideo) {
+                        return localVideo.mute();
+                    }
+                })
+                .catch(error => {
+                    logger.error('failed to switch back to local video', error);
+                    return this.useVideoStream(null).then(() => {
+                        // Still fail with the original err
+                        return Promise.reject(error);
+                    });
+                });
+        } else {
+            promise = this.useVideoStream(null);
+        }
+        return promise.then(
+            () => {
+                this.videoSwitchInProgress = false;
+            },
+            error => {
+                this.videoSwitchInProgress = false;
+                throw error;
+            });
+    },
+
     /**
      * Toggles between screensharing and camera video.
      * @param {boolean} [shareScreen]
@@ -1121,7 +1186,7 @@ export default {
      * 'window', etc.).
      * @return {Promise.<T>}
      */
-    toggleScreenSharing(shareScreen = !this.isSharingScreen, options = {}) {
+    toggleScreenSharing(options = {}) {
         if (this.videoSwitchInProgress) {
             return Promise.reject('Switch in progress.');
         }
@@ -1136,11 +1201,11 @@ export default {
             return Promise.reject('No screensharing in audio only mode');
         }
 
+        if (!this._untoggleScreenSharing) {
             this.videoSwitchInProgress = true;
             let externalInstallation = false;
-
-        if (shareScreen) {
-            const didHaveVideo = Boolean(this.localVideo);
+            const didHaveVideo = Boolean(localVideo);
+            const wasVideoMuted = this.videoMuted;
 
             return createLocalTracks({
                 desktopSharingSources: options.desktopSharingSources,
@@ -1168,22 +1233,26 @@ export default {
                     }
                 }
             }).then(([stream]) => {
+                // Stores the "untoggle" handler which remembers whether was
+                // there any video before and whether was it muted.
+                this._untoggleScreenSharing
+                    = this._turnScreenSharingOff
+                          .bind(this, didHaveVideo, wasVideoMuted);
                 DSExternalInstallationInProgress = false;
                 // close external installation dialog on success.
                 if(externalInstallation)
                     $.prompt.close();
-                if (didHaveVideo) {
                 stream.on(
                     TrackEvents.LOCAL_TRACK_STOPPED,
                     () => {
                         // If the stream was stopped during screen sharing
                         // session then we should switch back to video.
                         if (this.isSharingScreen){
-                                this.toggleScreenSharing(false);
+                            this._untoggleScreenSharing
+                                && this._untoggleScreenSharing();
                         }
                     }
                 );
-                }
                 return this.useVideoStream(stream);
             }).then(() => {
                 this.videoSwitchInProgress = false;
@@ -1200,7 +1269,17 @@ export default {
                     return Promise.reject(err);
                 }
 
-                this.toggleScreenSharing(false);
+                // Pawel: With this call I'm trying to preserve the original
+                // behaviour although it is not clear why would we "untoggle"
+                // on failure. I suppose it was to restore video in case there
+                // was some problem during "this.useVideoStream(desktopStream)".
+                // It's important to note that the handler will not be available
+                // if we fail early on trying to get desktop media (which makes
+                // sense, because the camera video is still being used, so
+                // nothing to "untoggle").
+                if (this._untoggleScreenSharing) {
+                    this._untoggleScreenSharing();
+                }
 
                 logger.error('failed to share local desktop', err);
 
@@ -1233,21 +1312,7 @@ export default {
                     dialogTitleKey, dialogTxt, false);
             });
         } else {
-            APP.remoteControl.receiver.stop();
-            return createLocalTracks(
-                { devices: ['video'] })
-            .then(
-                ([stream]) => this.useVideoStream(stream)
-            ).then(() => {
-                this.videoSwitchInProgress = false;
-                JitsiMeetJS.analytics.sendEvent(
-                    'conference.sharingDesktop.stop');
-                logger.log('sharing local video');
-            }).catch((err) => {
-                this.useVideoStream(null);
-                this.videoSwitchInProgress = false;
-                logger.error('failed to share local video', err);
-            });
+            return this._untoggleScreenSharing();
         }
     },
     /**