Fix: preserve original Figure DPI in pickle to prevent HiDPI doubling (macOSX)

lib/matplotlib/figure.py

@@ -3026,6 +3026,15 @@ def __getstate__(self):
         # add version information to the state
         state['__mpl_version__'] = mpl.__version__
 
+        # Ensure DPI stored in pickle is the unscaled, original DPI.
+        # When a HiDPI backend (e.g., macOSX) sets a device pixel ratio, the
+        # runtime Figure.dpi is scaled from the original value. If we persisted
+        # the scaled dpi here, unpickling and recreating the canvas would set
+        # _original_dpi from an already-scaled value and the backend would
+        # scale it again, effectively doubling the dpi on each cycle.
+        # Persisting the original dpi avoids cumulative scaling.
+        state["_dpi"] = getattr(self, "_original_dpi", self._dpi)
+
         # check whether the figure manager (if any) is registered with pyplot
         from matplotlib import _pylab_helpers
         if self.canvas.manager in _pylab_helpers.Gcf.figs.values():

lib/matplotlib/tests/test_pickle.py

@@ -237,3 +237,27 @@ def test_dynamic_norm():
 def test_vertexselector():
     line, = plt.plot([0, 1], picker=True)
     pickle.loads(pickle.dumps(VertexSelector(line)))
+
+
+def test_hidpi_pickle_unpickle_does_not_double_dpi():
+    # Emulate a HiDPI backend by setting device pixel ratio to 2. Verify that
+    # pickling and unpickling the figure does not cause the dpi to double on
+    # subsequent device pixel ratio application.
+    fig = plt.figure(dpi=100)
+    # Emulate HiDPI scaling applied by backend.
+    fig.canvas._set_device_pixel_ratio(2)
+    assert fig.dpi == 200
+
+    # First unpickle cycle should not change effective dpi when DPR=2 is
+    # applied again by the backend.
+    pf = pickle.dumps(fig, pickle.HIGHEST_PROTOCOL)
+    fig2 = pickle.loads(pf)
+    # Emulate backend reapplying DPR on new canvas.
+    fig2.canvas._set_device_pixel_ratio(2)
+    assert fig2.dpi == 200
+
+    # Repeated cycles should remain stable.
+    pf2 = pickle.dumps(fig2, pickle.HIGHEST_PROTOCOL)
+    fig3 = pickle.loads(pf2)
+    fig3.canvas._set_device_pixel_ratio(2)
+    assert fig3.dpi == 200