ipython/ipython — pull request #7097 by jasongrout (2014-12-03 23:58:04+00:00)
Title: Change the semantics of the widget visible attribute to allow for visiblility: hidden

Now, visible can have three values:
* True - show the widget
* False - hide the widget, but show the empty space
* None - hide the widget and collapse the empty space

Previously, None was not allowed, and False collapsed the empty space.