matplotlib · jklymak · Dec 18, 2020 · Dec 16, 2020
diff --git a/lib/matplotlib/backends/backend_qt5.py b/lib/matplotlib/backends/backend_qt5.py
@@ -227,12 +227,8 @@ def __init__(self, figure):
         self._update_figure_dpi()
         # In cases with mixed resolution displays, we need to be careful if the
         # dpi_ratio changes - in this case we need to resize the canvas
-        # accordingly. We could watch for screenChanged events from Qt, but
-        # the issue is that we can't guarantee this will be emitted *before*
-        # the first paintEvent for the canvas, so instead we keep track of the
-        # dpi_ratio value here and in paintEvent we resize the canvas if
-        # needed.
-        self._dpi_ratio_prev = None
+        # accordingly.
+        self._dpi_ratio_prev = self._dpi_ratio
 
         self._draw_pending = False
         self._is_drawing = False
@@ -253,12 +249,9 @@ def _update_figure_dpi(self):
     def _dpi_ratio(self):
         return _devicePixelRatioF(self)
 
-    def _update_dpi(self):
-        # As described in __init__ above, we need to be careful in cases with
-        # mixed resolution displays if dpi_ratio is changing between painting
-        # events.
-        # Return whether we triggered a resizeEvent (and thus a paintEvent)
-        # from within this function.
+    def _update_pixel_ratio(self):
+        # We need to be careful in cases with mixed resolution displays if
+        # dpi_ratio changes.
         if self._dpi_ratio != self._dpi_ratio_prev:
             # We need to update the figure DPI.
             self._update_figure_dpi()
@@ -270,8 +263,20 @@ def _update_dpi(self):
             self.resizeEvent(event)
             # resizeEvent triggers a paintEvent itself, so we exit this one
             # (after making sure that the event is immediately handled).
-            return True
-        return False
+
+    def _update_screen(self, screen):
+        # Handler for changes to a window's attached screen.
+        self._update_pixel_ratio()
+        if screen is not None:
+            screen.physicalDotsPerInchChanged.connect(self._update_pixel_ratio)
+            screen.logicalDotsPerInchChanged.connect(self._update_pixel_ratio)
+
+    def showEvent(self, event):
+        # Set up correct pixel ratio, and connect to any signal changes for it,
+        # once the window is shown (and thus has these attributes).
+        window = self.window().windowHandle()
+        window.screenChanged.connect(self._update_screen)
+        self._update_screen(window.screen())
 
     def get_width_height(self):
         w, h = FigureCanvasBase.get_width_height(self)
@@ -364,10 +369,6 @@ def keyReleaseEvent(self, event):
             FigureCanvasBase.key_release_event(self, key, guiEvent=event)
 
     def resizeEvent(self, event):
-        # _dpi_ratio_prev will be set the first time the canvas is painted, and
-        # the rendered buffer is useless before anyways.
-        if self._dpi_ratio_prev is None:
-            return
         w = event.size().width() * self._dpi_ratio
         h = event.size().height() * self._dpi_ratio
         dpival = self.figure.dpi

diff --git a/lib/matplotlib/backends/backend_qt5agg.py b/lib/matplotlib/backends/backend_qt5agg.py
@@ -27,9 +27,6 @@ def paintEvent(self, event):
         In Qt, all drawing should be done inside of here when a widget is
         shown onscreen.
         """
-        if self._update_dpi():
-            # The dpi update triggered its own paintEvent.
-            return
         self._draw_idle()  # Only does something if a draw is pending.
 
         # If the canvas does not have a renderer, then give up and wait for

diff --git a/lib/matplotlib/backends/backend_qt5cairo.py b/lib/matplotlib/backends/backend_qt5cairo.py
@@ -17,7 +17,6 @@ def draw(self):
         super().draw()
 
     def paintEvent(self, event):
-        self._update_dpi()
         dpi_ratio = self._dpi_ratio
         width = int(dpi_ratio * self.width())
         height = int(dpi_ratio * self.height())

diff --git a/lib/matplotlib/tests/test_backend_qt.py b/lib/matplotlib/tests/test_backend_qt.py
@@ -158,32 +158,38 @@ def on_key_press(event):
 
 
 @pytest.mark.backend('Qt5Agg')
-def test_dpi_ratio_change():
+def test_pixel_ratio_change():
     """
-    Make sure that if _dpi_ratio changes, the figure dpi changes but the
+    Make sure that if the pixel ratio changes, the figure dpi changes but the
     widget remains the same physical size.
     """
 
-    prop = 'matplotlib.backends.backend_qt5.FigureCanvasQT._dpi_ratio'
-
-    with mock.patch(prop, new_callable=mock.PropertyMock) as p:
-
+    prop = 'matplotlib.backends.backend_qt5.FigureCanvasQT.devicePixelRatioF'
+    with mock.patch(prop) as p:
         p.return_value = 3
 
         fig = plt.figure(figsize=(5, 2), dpi=120)
         qt_canvas = fig.canvas
         qt_canvas.show()
 
-        from matplotlib.backends.backend_qt5 import qApp
+        def set_pixel_ratio(ratio):
+            p.return_value = ratio
+            # Make sure the mocking worked
+            assert qt_canvas._dpi_ratio == ratio
 
-        # Make sure the mocking worked
-        assert qt_canvas._dpi_ratio == 3
+            # The value here doesn't matter, as we can't mock the C++ QScreen
+            # object, but can override the functional wrapper around it.
+            # Emitting this event is simply to trigger the DPI change handler
+            # in Matplotlib in the same manner that it would occur normally.
+            screen.logicalDotsPerInchChanged.emit(96)
 
-        size = qt_canvas.size()
+            qt_canvas.draw()
+            qt_canvas.flush_events()
 
         qt_canvas.manager.show()
-        qt_canvas.draw()
-        qApp.processEvents()
+        size = qt_canvas.size()
+        screen = qt_canvas.window().windowHandle().screen()
+        set_pixel_ratio(3)
 
         # The DPI and the renderer width/height change
         assert fig.dpi == 360
@@ -196,17 +202,7 @@ def test_dpi_ratio_change():
         assert qt_canvas.get_width_height() == (600, 240)
         assert (fig.get_size_inches() == (5, 2)).all()
 
-        p.return_value = 2
-
-        assert qt_canvas._dpi_ratio == 2
-
-        qt_canvas.draw()
-        qApp.processEvents()
-        # this second processEvents is required to fully run the draw.
-        # On `update` we notice the DPI has changed and trigger a
-        # resize event to refresh, the second processEvents is
-        # required to process that and fully update the window sizes.
-        qApp.processEvents()
+        set_pixel_ratio(2)
 
         # The DPI and the renderer width/height change
         assert fig.dpi == 240
@@ -219,17 +215,7 @@ def test_dpi_ratio_change():
         assert qt_canvas.get_width_height() == (600, 240)
         assert (fig.get_size_inches() == (5, 2)).all()
 
-        p.return_value = 1.5
-
-        assert qt_canvas._dpi_ratio == 1.5
-
-        qt_canvas.draw()
-        qApp.processEvents()
-        # this second processEvents is required to fully run the draw.
-        # On `update` we notice the DPI has changed and trigger a
-        # resize event to refresh, the second processEvents is
-        # required to process that and fully update the window sizes.
-        qApp.processEvents()
+        set_pixel_ratio(1.5)
 
         # The DPI and the renderer width/height change
         assert fig.dpi == 180