All these tests should build a reasonable frame tree without asserting.
  
    This text is in a positioned box with scrollbars.
    
      Scroll to see this text. (#1)
    
  
    This text is in a box with scrollbars.
    
      This text should be visible on the main page. (#1)
    
  
    This text is in a positioned box with columns, without scrollbars.
    
      This text should be visible on the main page. (#2)
    
  
    This text is in a positioned box with scrollbars and columns.
    
      Scroll to see this text. (#2)
    
  
    This text is in a box with scrollbars and columns.
    
      This text should be visible on the main page. (#3)