Did this work before and we broke it, or did it never work? There were some changes to colorbars, but I'm not sure why they would cause this to happen. Thanks.

I'm not sure but I can try running my code using older versions of matplotlib. What version do you think might be the culprit?

Try 3.4.2. But if you don't know if this ever worked it probably never did. My guess would be there is a typo in the code that makes the hatch. Maybe east to fix if you grep hatch in the colorbar.py

The bug still happens in v3.4.2. I did find a solution that fixes the issue without messing up vertical colorbars with hatches. I works in both 3.5.2 and 3.4.2
In _add_solids_patches, change the indices for creating xy from

    def _add_solids_patches(self, X, Y, C, mappable):
        hatches = mappable.hatches * len(C)  # Have enough hatches.
        patches = []
        for i in range(len(X) - 1):
           xy = np.array([[X[i, 0], Y[i, 0]],
                           [X[i, 1], Y[i, 0]],
                           [X[i + 1, 1], Y[i + 1, 0]],
                           [X[i + 1, 0], Y[i + 1, 1]]])

To this

    def _add_solids_patches(self, X, Y, C, mappable):
        hatches = mappable.hatches * len(C)  # Have enough hatches.
        patches = []
        for i in range(len(X) - 1):
           xy = np.array([[X[i, 0], Y[i, 1]],  # this is the only line that needs a change
                           [X[i, 1], Y[i, 0]],
                           [X[i + 1, 1], Y[i + 1, 0]],
                           [X[i + 1, 0], Y[i + 1, 1]]])

Yep, that looks like an indexing bug... If you have a patch, feel free to try a pull request. Maybe also add a test to test_colorbar.py? It would require an image comparison. https://matplotlib.org/stable/devel/index.html for more guidance, also feel free to ask at https://gitter.im/matplotlib/matplotlib

I was about to implement this, but I found another issue as well. A slightly different setting gave this (after the patch above was introduced):

Clearly the colors and hatches are off by one.

That seems to be a problem for vertical as well (and maybe easier to see there):

The same thing can be seen in the original picture although not as obvious there. Rather, there are eight different levels in both the contour and the colorbar, but the left extended level in the colorbar is not matching the darkest in the contour. However, of the three lightest/right-most three levels in the contour, only two of them are in the colorbar.

Not sure if it is worthwhile doing a patch to get the rectangle correct as it probably doesn't make sense to add a test image since the colorbar in itself doesn't match the contour-levels.

The problem is related to extend. Without it, it is all good.

So, _add_solids_patches duplicates the hatches:

The extend patches picks the first and last hatch pattern of the original list: