How to change controlbar to full screen?

function onFullscreen(e:FullScreenEvent):void {
      if (e.fullScreen) {
          
          // switch fullscreen buttons
          mcVideoControls.btnFullscreenOn.visible = false;
          mcVideoControls.btnFullscreenOff.visible = true;
                         
          // bottom center align controls
          mcVideoControls.x = (Capabilities.screenResolutionX - 480) / 2;
          mcVideoControls.y = (Capabilities.screenResolutionY - 36);
          btnMidPlay.x = stage.stageWidth / 2;
          btnMidPlay.y = stage.stageHeight / 2;
                    
          // size up video display
          vidDisplay.height      = (Capabilities.screenResolutionY);
          vidDisplay.width      = vidDisplay.height * 4 / 3;
          vidDisplay.x          = (Capabilities.screenResolutionX - vidDisplay.width) / 2;
          mcVideoControls.width=stage.stageWidth;
     } else {
          // switch fullscreen buttons
        mcVideoControls.btnFullscreenOn.visible = true;
          mcVideoControls.btnFullscreenOff.visible = false;

          // reset controls position
          mcVideoControls.x = 0;
          mcVideoControls.y = 325;
          btnMidPlay.x = 240;
                btnMidPlay.y = 180;
          // reset video display
          vidDisplay.y = 0;
          vidDisplay.x = 0;
          vidDisplay.width = 480;
          vidDisplay.height = 360;
    }
}

The above will change the width of the single movieclip, but it is not the way to do that, you should seperate your inner clips and make your controls to certain width and reposition the controls within that. otherwise you will face the issue after implementing the above.

You are changing the position of mcVideoControls but not changing the size. You need to resize it and reposition the buttons inside.