I'm guessing you mean the screen has the video player on the left & the music notation on the right.  But in the upper right corner of the notation there is a button to Popup or Hide.  The problem is that once the user clicks either of those, the notation is in a separate window or gone, replaced by only the video player.   There is no button to reverse the process.

The notation is still available below the video player (click the Notation tab).  Or you simply refresh the page (F5 on a WinOS).  That resets the page for me & the screen is once again the video player on the left & the notation on the right. 

Hope that helps.