difference in plt.gca() behaviour between inline and notebook backends

I have one notebook cell that creates a 3D axes, which I want to plot into in the next cell.

## With the inline backend

we see that the "current plot" has been forgotten, and `plt.gca()` creates a new 2D plot.

## With the notebook backend

The behaviour is different

and `plt.gca()` remembers the 3D plot across the cell boundaries.

## Expected behaviour

The same in both cases, ideally the remembering the current plot across the cell boundary. I'm not sure which behaviour is "correct" but the difference is a problem when I'm running code in an ipython kernel from a client.