Panning/zooming a wcs plot broken with (unreleased) Matplotlib 3.1.0

Description

Zooming or panning a plot with a wcs projection is broken with the current master branch of Maptlotlib, that should shortly be released as version 3.1.0. I am aware that this is not the best bug report, but I thought I’d leave it anyway as a placeholder for fixing at some point.

This might be an upstream Maptlotlib issue.

Steps to Reproduce

1. Install Matplotlib master branch
2. Run the following code:

from matplotlib import pyplot as plt
from astropy.io import fits
from astropy.wcs import WCS
from astropy.utils.data import get_pkg_data_filename

filename = get_pkg_data_filename('tutorials/FITS-images/HorseHead.fits')

hdu = fits.open(filename)[0]
wcs = WCS(hdu.header)

fig = plt.figure()
fig.add_subplot(111, projection=wcs)
plt.imshow(hdu.data)
plt.show()

1. Try to zoom/pan

Error message is:

Traceback (most recent call last):
  File "/Users/dstansby/miniconda3/lib/python3.7/site-packages/numpy/lib/shape_base.py", line 743, in array_split
    Ntotal = ary.shape[axis]
AttributeError: 'NoneType' object has no attribute 'shape'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/dstansby/github/matplotlib/lib/matplotlib/cbook/__init__.py", line 216, in process
    func(*args, **kwargs)
  File "/Users/dstansby/github/matplotlib/lib/matplotlib/backend_bases.py", line 2791, in press_pan
    if (x is not None and y is not None and a.in_axes(event) and
  File "/Users/dstansby/github/matplotlib/lib/matplotlib/axes/_base.py", line 2126, in in_axes
    return self.patch.contains(mouseevent)[0]
  File "/Users/dstansby/github/matplotlib/lib/matplotlib/patches.py", line 144, in contains
    Path, np.split(vertices, idxs), np.split(codes, idxs))), {}
  File "/Users/dstansby/miniconda3/lib/python3.7/site-packages/numpy/lib/shape_base.py", line 850, in split
    res = array_split(ary, indices_or_sections, axis)
  File "/Users/dstansby/miniconda3/lib/python3.7/site-packages/numpy/lib/shape_base.py", line 745, in array_split
    Ntotal = len(ary)
TypeError: object of type 'NoneType' has no len()
Traceback (most recent call last):
  File "/Users/dstansby/miniconda3/lib/python3.7/site-packages/numpy/lib/shape_base.py", line 743, in array_split
    Ntotal = ary.shape[axis]
AttributeError: 'NoneType' object has no attribute 'shape'

[dstansby]

Doing a bit of investigating, this bisects to https://github.com/matplotlib/matplotlib/pull/10776 in Matplotlib

[dstansby]

I’m using the QT5Agg backend.