PR summary

Discovered when I was trying to wrap my head around #30089. Given

import matplotlib.pyplot as plt

mosaic1 = "AC;BC"
mosaic2 = "ACDE;BCDE"

for mosaic in mosaic1, mosaic2:
    fig, ax_dir = plt.subplot_mosaic(mosaic, layout='constrained', facecolor='skyblue')
    fig.get_layout_engine().set(h_pad=0.2)
    
plt.show()

On main, the gap between the rows of the first column is larger when there are more other columns

Currently the "submerged margins" function works out the required size of the margin by summing the main margin and the "cb" margin (which includes our initial pad). If I have understood, we then apply that required size to only the main margin. So the "cb" margin size gets added on each time we pass through the loop.

Here I just subtract the "cb" values we started with from the target margin size. So the total margin should be what we are aiming for. So the above code now gives:

Given that some test images changed, I'm unsure if this would count as an API change.

PR checklist

• [n/a] "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• [n/a] Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines