Closes#30076

If a figure had subfigures, the submerged margin algorithm (that tries to deal with margins that are not shared across all axes) would get applied toall axes in the figure, including any that had already been taken care of in a subfigure. This lead to the erroneous behaviour described in#30076.

New version does not apply the algorithm if the axes has already been dealt with in a subfigure.

importmatplotlib.pyplotaspltdeftest(layout,labelled=False):ll=labelledfig=plt.figure(figsize=(4,9),layout=layout)fig.suptitle(f'Labelled ={notlabelled}',fontsize='small')figures=fig.subfigures(4,1)forfinfigures.flatten():gs=f.add_gridspec(2,2)foriinrange(2):ax=f.add_subplot(gs[i,0])ax.plot()ifnotlabelled:ax.set_xlabel('BOOO',fontsize='large')labelled=Truef.add_subplot(gs[:,1]).plot()fig.savefig(f'fig_{notll}.png')test("constrained")#test("constrained", labelled=True)plt.show()

This PR

now yields:

On main

Test just makes sure that the appropriate subplots are the same height now. The test fails on main.