Merge pull request matplotlib#30106 from meeseeksmachine/auto-backport-of-pr-30089-on-v3.10.x

Backport PR matplotlib#30089 on branch v3.10.x (FIX: fix submerged margins algorithm being applied twice)

Author: QuLogic

lib/matplotlib/_constrained_layout.py

@@ -521,11 +521,13 @@ def match_submerged_margins(layoutgrids, fig):
     See test_constrained_layout::test_constrained_layout12 for an example.
     """
 
+    axsdone = []
     for sfig in fig.subfigs:
-        match_submerged_margins(layoutgrids, sfig)
+        axsdone += match_submerged_margins(layoutgrids, sfig)
 
     axs = [a for a in fig.get_axes()
-           if a.get_subplotspec() is not None and a.get_in_layout()]
+           if (a.get_subplotspec() is not None and a.get_in_layout() and
+               a not in axsdone)]
 
     for ax1 in axs:
         ss1 = ax1.get_subplotspec()
@@ -592,6 +594,8 @@ def match_submerged_margins(layoutgrids, fig):
             for i in ss1.rowspan[:-1]:
                 lg1.edit_margin_min('bottom', maxsubb, cell=i)
 
+    return axs
+
 
 def get_cb_parent_spans(cbax):
     """

lib/matplotlib/tests/test_constrainedlayout.py

@@ -721,3 +721,23 @@ def test_layout_leak():
     gc.collect()
     assert not any(isinstance(obj, mpl._layoutgrid.LayoutGrid)
                    for obj in gc.get_objects())
+
+
+def test_submerged_subfig():
+    """
+    Test that the submerged margin logic does not get called multiple times
+    on same axes if it is already in a subfigure
+    """
+    fig = plt.figure(figsize=(4, 5), layout='constrained')
+    figures = fig.subfigures(3, 1)
+    axs = []
+    for f in figures.flatten():
+        gs = f.add_gridspec(2, 2)
+        for i in range(2):
+            axs += [f.add_subplot(gs[i, 0])]
+            axs[-1].plot()
+        f.add_subplot(gs[:, 1]).plot()
+    fig.draw_without_rendering()
+    for ax in axs[1:]:
+        assert np.allclose(ax.get_position().bounds[-1],
+                           axs[0].get_position().bounds[-1], atol=1e-6)