visibilityState property
      
      DocumentVisibilityState
      get
      visibilityState
      
    
    
@AvailableInWorkers("service")
The visibilityState read-only property of the
WindowClient interface indicates the visibility of the current client.
This value can be one of "hidden", "visible", or
"prerender".
Implementation
external DocumentVisibilityState get visibilityState;