This seems OK. However, can we make sure it works when there actually is a colorbar? (cb is shorthand for the colorbar margins)

Something like this?

import matplotlib.pyplot as plt

mosaic = "AABBCC;DDDEEE"

fig, ax_dir = plt.subplot_mosaic(mosaic, layout='constrained', facecolor='skyblue')

cf = ax_dir['A'].contourf([[0, 1], [2, 3]])
fig.colorbar(cf)
    
plt.show()

Hmm, if I dont' have the submerged axes then I get

But I guess we want the middle axes centred on the bottom two and the same size as the two to the right, so maybe this PR good?

Rebased and got linting failures I don't understand. They are about rst files.

Does this need a test for a case with a colorbar, and if so is an image comparison test based on #30108 (comment) enough?

I think so. I may have limited availability but feel free to ping for a review after 13 may. Or other folks can merge. I think this approach is good

@QuLogic can tihs still go into 3.11?

Because it's a visual change, do we want to mention it in the release notes?

I think we may still have some room for it.

I can write a release note if desired. Should I put it in the usual place in this PR? Or at this point is it simpler to just provide some text to go straight into #31642?

Added the changenote here as I figured I needed to check it renders OK.

Note that given

import matplotlib.pyplot as plt

plt.subplot_mosaic('AC;BC', layout='constrained')
plt.savefig('simple_mosaic.png')

plt.subplots(2, 2, layout='constrained')
plt.savefig('simple_subplots.png')

This PR make the spacing for the left column the same in both figures. So I think the smaller spacing can be considered a bug fix. Of course it may still be surprising to some users so worth flagging in the notes.