BUG: Refresh 3D axis projection before tightbbox to fix constrained layout overlap (GH#31277)

tinezivic · tinezivic · commit 4c805f7f1fa4 · 2026-04-22T22:32:41.000+02:00
Axis3D.get_tightbbox() read stale tick-label (x, y) positions that were
last set during the previous draw() call. When constrained_layout queries
the bounding boxes before any draw has occurred, the positions are
uninitialised, causing the 3D subplot's reported bbox to be too small.
Adjacent subplots then overlap the 3D tick labels.

Fix: at the start of Axis3D.get_tightbbox(), refresh the projection
matrix (M / invM) via apply_aspect() and get_proj(), then call
self.draw() inside renderer._draw_disabled() to update the tick-label
positions without producing any visible output. The rest of the method
already reads those positions correctly.

Two regression tests are added:
- test_axis_get_tightbbox_before_draw_matches_after_draw: the bbox
  returned before any draw() must match the one returned after draw().
- test_constrained_layout_3d_no_overlap_with_subplot_title: with a 3D
  subplot above a 2D subplot in constrained_layout, the 3D tight bbox
  bottom must not overlap the 2D subplot title.
diff --git a/lib/mpl_toolkits/mplot3d/axis3d.py b/lib/mpl_toolkits/mplot3d/axis3d.py
@@ -684,6 +684,23 @@ def get_tightbbox(self, renderer=None, *, for_layout_only=False):
         # docstring inherited
         if not self.get_visible():
             return
+        if renderer is None:
+            renderer = self.get_figure(root=True)._get_renderer()
+
+        # Ensure the axes aspect and projection matrix are up to date so that
+        # the tick-label positions computed below match what draw() will use.
+        locator = self.axes.get_axes_locator()
+        self.axes.apply_aspect(
+            locator(self.axes, renderer) if locator else None)
+        self.axes.M = self.axes.get_proj()
+        self.axes.invM = np.linalg.inv(self.axes.M)
+
+        # The 2D positions of 3D ticks and labels are updated during draw()
+        # based on the current projection. Refresh that state without
+        # rendering so the bbox query sees the same geometry as the real draw.
+        with renderer._draw_disabled():
+            self.draw(renderer)
+
         # We have to directly access the internal data structures
         # (and hope they are up to date) because at draw time we
         # shift the ticks and their labels around in (x, y) space
diff --git a/lib/mpl_toolkits/mplot3d/tests/test_axes3d.py b/lib/mpl_toolkits/mplot3d/tests/test_axes3d.py
@@ -2780,6 +2780,49 @@ def test_axis_get_tightbbox_includes_offset_text():
             f"bbox.y1 ({bbox.y1}) should be >= offset_bbox.y1 ({offset_bbox.y1})"
 
 
+def test_axis_get_tightbbox_before_draw_matches_after_draw():
+    # Regression test for GH#31277.
+    # Axis3D.get_tightbbox() must refresh projected tick positions so that the
+    # bbox it returns is the same whether or not draw() has already been called.
+    fig = plt.figure()
+    ax = fig.add_subplot(projection='3d')
+
+    axes = [ax.xaxis, ax.yaxis, ax.zaxis]
+
+    renderer = fig.canvas.get_renderer()
+    bboxes_before = {
+        axis.axis_name: axis.get_tightbbox(renderer).extents.copy()
+        for axis in axes
+    }
+
+    fig.canvas.draw()
+    renderer = fig.canvas.get_renderer()
+    for axis in axes:
+        np.testing.assert_allclose(
+            bboxes_before[axis.axis_name],
+            axis.get_tightbbox(renderer).extents,
+            atol=1,
+            err_msg=f"{axis.axis_name}: tightbbox before draw differs from after draw",
+        )
+
+
+def test_constrained_layout_3d_no_overlap_with_subplot_title():
+    # Regression test for GH#31277.
+    # When using constrained_layout, 3D tick labels must not overlap the title
+    # of an adjacent 2D subplot.
+    fig = plt.figure(constrained_layout=True)
+    ax1 = fig.add_subplot(2, 1, 1, projection='3d')
+    ax1.set_title('3D Plot')
+    ax2 = fig.add_subplot(2, 1, 2)
+    ax2.set_title('2D Plot')
+
+    fig.canvas.draw()
+    renderer = fig.canvas.get_renderer()
+
+    assert ax1.get_tightbbox(renderer).y0 >= ax2.title.get_window_extent(renderer).y1, \
+        "3D subplot tick labels overlap the title of the 2D subplot below"
+
+
 def test_ctrl_rotation_snaps_to_5deg():
     fig = plt.figure()
     ax = fig.add_subplot(projection='3d')
