old_contents: import atexit
  import os
  
  _xvfb = None
  
  def start_xvfb():
    from xvfbwrapper import Xvfb
    global _xvfb
    if "DISPLAY" in os.environ:
      del os.environ["DISPLAY"]
    _xvfb = Xvfb()
    _xvfb.start()
    atexit.register(_xvfb.stop)
  
  def stop_xvfb():
    global _xvfb
    if "DISPLAY" in os.environ:
      _xvfb.stop()
      del os.environ["DISPLAY"]
  
new_contents: import atexit
  import os
  
  _xvfb = None
  
  def start_xvfb():
    from xvfbwrapper import Xvfb
    global _xvfb
    if "DISPLAY" in os.environ:
      del os.environ["DISPLAY"]
    _xvfb = Xvfb()
    _xvfb.start()
    atexit.register(_xvfb.stop)
  
  def stop_xvfb():
    global _xvfb
    _xvfb.stop()
  
subject: fix xvfb_stop