hide header-bar when header is hidden
hide header-bar when header is hidden

also reduce the amount of CSS used to accomplish the 1px line (no negative margins)

SylvainCorlay/ipywidgets,jupyter-widgets/ipywidgets,cornhundred/ipywidgets,ipython/ipywidgets,cornhundred/ipywidgets,cornhundred/ipywidgets,ipython/ipywidgets,cornhundred/ipywidgets,SylvainCorlay/ipywidgets,jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,cornhundred/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets