[Bugfix] Preserve original DPI in pickled figures to avoid HiDPI doubling (swev-id: matplotlib__matplotlib-23476)

lib/matplotlib/figure.py

@@ -3043,6 +3043,12 @@ def __setstate__(self, state):
 
         self.__dict__ = state
 
+        # Preserve the original DPI that may differ from the figure DPI on
+        # HiDPI backends.  FigureCanvasBase will overwrite ``_original_dpi``
+        # when it reattaches the canvas, so capture it first and restore it
+        # afterwards.
+        original_dpi = getattr(self, "_original_dpi", self.dpi)
+
         # re-initialise some of the unstored state information
         FigureCanvasBase(self)  # Set self.canvas.
 
@@ -3056,6 +3062,8 @@ def __setstate__(self, state):
             pylab_helpers.Gcf._set_new_active_manager(mgr)
             plt.draw_if_interactive()
 
+        self._original_dpi = original_dpi
+
         self.stale = True
 
     def add_axobserver(self, func):

lib/matplotlib/tests/test_pickle.py

@@ -217,6 +217,25 @@ def test_unpickle_canvas():
     assert fig2.canvas is not None
 
 
+def test_hidpi_pickle_unpickle_does_not_double_dpi():
+    fig = plt.figure(dpi=100)
+
+    fig.canvas._set_device_pixel_ratio(2)
+    assert fig.dpi == pytest.approx(200)
+    assert getattr(fig, "_original_dpi", None) == pytest.approx(100)
+
+    buffer = BytesIO()
+    pickle.dump(fig, buffer, pickle.HIGHEST_PROTOCOL)
+    buffer.seek(0)
+
+    restored = pickle.load(buffer)
+    assert restored.dpi == pytest.approx(200)
+
+    restored.canvas._set_device_pixel_ratio(2)
+    assert restored.dpi == pytest.approx(200)
+    assert getattr(restored, "_original_dpi", None) == pytest.approx(100)
+
+
 def test_mpl_toolkits():
     ax = parasite_axes.host_axes([0, 0, 1, 1])
     assert type(pickle.loads(pickle.dumps(ax))) == parasite_axes.HostAxes