fix(screen-sharing) Self view of SC sized correctly initially (#13992)

react/features/large-video/components/LargeVideo.web.tsx

@@ -228,16 +228,17 @@ class LargeVideo extends Component<IProps> {
                       * another container for the background and the
                       * largeVideoWrapper in order to hide/show them.
                       */}
+                    { _displayScreenSharingPlaceholder ? <ScreenSharePlaceholder /> : <></>}
                     <div
                         id = 'largeVideoWrapper'
                         onTouchEnd = { this._onDoubleTap }
                         ref = { this._wrapperRef }
                         role = 'figure' >
-                        { _displayScreenSharingPlaceholder ? <ScreenSharePlaceholder /> : <video
+                        <video
                             autoPlay = { !_noAutoPlayVideo }
                             id = 'largeVideo'
                             muted = { true }
-                            playsInline = { true } /* for Safari on iOS to work */ /> }
+                            playsInline = { true } /* for Safari on iOS to work */ />
                     </div>
                 </div>
                 { interfaceConfig.DISABLE_TRANSCRIPTION_SUBTITLES

react/features/large-video/components/ScreenSharePlaceholder.web.tsx

@@ -15,7 +15,10 @@ const useStyles = makeStyles()(theme => {
             display: 'flex',
             justifyContent: 'center',
             alignItems: 'center',
-            position: 'absolute'
+            position: 'absolute',
+            top: 0,
+            left: 0,
+            zIndex: 2
         },
         content: {
             display: 'flex',