matplotlib/matplotlib — pull request #3360 by tacaswell (2014-08-10 23:18:24+00:00)
Title: BUG : modified logic on starting qApp

when working in ipython, it seems that _create_qApp is not called
until the first time that a figure is made, but ipython has already
started up the QApplication for us (I think) so that we never set
backend_qt5 to be non-None. This seems to mostly be ok, but breaks
`plt.pause`.

I don't have time right now to track down the when this bug came in and if this is something we changed or something Ipython changed.