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

tinezivic · tinezivic · commit b55e1e16310f · 2026-04-24T00:47:00.000+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()
+
+        # For constrained_layout, the layout engine queries tightbbox before
+        # any draw() call, so tick 2D positions have not been set yet and the
+        # bbox would be degenerate.  Do a no-output draw to initialize them.
+        # M must be computed first if it hasn't been set yet.
+        # The flag is per-axis so each of xaxis/yaxis/zaxis is initialised once.
+        if (not getattr(self, '_tightbbox_initialized', False)
+                and self.get_figure(root=True).get_constrained_layout()):
+            if self.axes.M is None:
+                self.axes.M = self.axes.get_proj()
+                self.axes.invM = np.linalg.inv(self.axes.M)
+            with renderer._draw_disabled():
+                self.draw(renderer)
+            self._tightbbox_initialized = True
+
         # 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/baseline_images/test_axes3d/stem3d.png b/lib/mpl_toolkits/mplot3d/tests/baseline_images/test_axes3d/stem3d.png
diff --git a/lib/mpl_toolkits/mplot3d/tests/test_axes3d.py b/lib/mpl_toolkits/mplot3d/tests/test_axes3d.py
@@ -2780,6 +2780,39 @@ 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_includes_ticklabels():
+    # Regression test for GH#31277.
+    # Axis3D.get_tightbbox() must return a non-degenerate bbox before draw()
+    # has been called when constrained_layout is active, so that the layout
+    # engine can account for 3D tick labels.
+    fig = plt.figure(constrained_layout=True)
+    ax = fig.add_subplot(projection='3d')
+
+    renderer = fig.canvas.get_renderer()
+    for axis in [ax.xaxis, ax.yaxis, ax.zaxis]:
+        bbox = axis.get_tightbbox(renderer)
+        assert bbox is not None, f"{axis.axis_name}: get_tightbbox returned None"
+        assert bbox.width > 0 and bbox.height > 0, \
+            f"{axis.axis_name}: tightbbox has zero area before 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')
