qt: Use better devicePixelRatio event to refresh scaling

With Qt 6.6, there is an event on the window that signals when the
devicePixelRatio has changed. This is better than before when we had to
rely on the underlying `QScreen`, which doesn't correctly refresh when a
fractional scale is used.

Fixes matplotlib#30218

Author: QuLogic

lib/matplotlib/backends/backend_qt.py

@@ -262,12 +262,21 @@ def _update_screen(self, screen):
             screen.physicalDotsPerInchChanged.connect(self._update_pixel_ratio)
             screen.logicalDotsPerInchChanged.connect(self._update_pixel_ratio)
 
+    def eventFilter(self, source, event):
+        if event.type() == QtCore.QEvent.Type.DevicePixelRatioChange:
+            self._update_pixel_ratio()
+        return super().eventFilter(source, event)
+
     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())
+        current_version = tuple(int(x) for x in QtCore.qVersion().split('.', 2)[:2])
+        if current_version >= (6, 6):
+            window.installEventFilter(self)
+        else:
+            window.screenChanged.connect(self._update_screen)
+            self._update_screen(window.screen())
 
     def set_cursor(self, cursor):
         # docstring inherited

lib/matplotlib/tests/test_backend_qt.py

@@ -137,7 +137,7 @@ def on_key_press(event):
 
 
 @pytest.mark.backend('QtAgg', skip_on_importerror=True)
-def test_device_pixel_ratio_change():
+def test_device_pixel_ratio_change(qt_core):
     """
     Make sure that if the pixel ratio changes, the figure dpi changes but the
     widget remains the same logical size.
@@ -154,11 +154,19 @@ def test_device_pixel_ratio_change():
         def set_device_pixel_ratio(ratio):
             p.return_value = ratio
 
-            # 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)
+            window = qt_canvas.window().windowHandle()
+            current_version = tuple(
+                int(x) for x in qt_core.qVersion().split('.', 2)[:2])
+            if current_version >= (6, 6):
+                qt_core.QCoreApplication.sendEvent(
+                    window,
+                    qt_core.QEvent(qt_core.QEvent.Type.DevicePixelRatioChange))
+            else:
+                # 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.
+                window.screen().logicalDotsPerInchChanged.emit(96)
 
             qt_canvas.draw()
             qt_canvas.flush_events()
@@ -168,7 +176,6 @@ def set_device_pixel_ratio(ratio):
 
         qt_canvas.manager.show()
         size = qt_canvas.size()
-        screen = qt_canvas.window().windowHandle().screen()
         set_device_pixel_ratio(3)
 
         # The DPI and the renderer width/height change