The DocumentPictureInPictureEvent interface of the Document Picture-in-Picture API  is the event object for the enter  event, which fires when the Picture-in-Picture window is opened.
    Event          DocumentPictureInPictureEvent     
 
Inherits methods from its parent, Event . 
 
Inherits properties from its parent, Event . 
  
window  Read only    Experimental   
   Returns a Window  instance representing the browsing context inside the DocumentPictureInPicture window the event was fired on.
    
 
documentPictureInPicture. addEventListener ( "enter" ,  ( event )  =>  { 
  const  pipWindow =  event. window; 
  console. log ( "Video player has entered the pip window" ) ; 
  const  pipMuteButton =  pipWindow. document. createElement ( "button" ) ; 
  pipMuteButton. textContent =  "Mute" ; 
  pipMuteButton. addEventListener ( "click" ,  ( )  =>  { 
    const  pipVideo =  pipWindow. document. querySelector ( "#video" ) ; 
    if  ( ! pipVideo. muted)  { 
      pipVideo. muted =  true ; 
      pipMuteButton. textContent =  "Unmute" ; 
    }  else  { 
      pipVideo. muted =  false ; 
      pipMuteButton. textContent =  "Mute" ; 
    } 
  } ) ; 
  pipWindow. document. body. append ( pipMuteButton) ; 
} ) ;