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 +262,22 @@ 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 source is self.window().windowHandle():
+            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