Support fractional HiDPI in GTK4 backend

Since GTK 4.12, fractional HiDPI is handled, but with a separate
property on the backing surface due to it being a different type.

Author: QuLogic

lib/matplotlib/backends/backend_gtk4.py

@@ -30,6 +30,7 @@
 )
 
 _GOBJECT_GE_3_47 = gi.version_info >= (3, 47, 0)
+_GTK_GE_4_12 = Gtk.check_version(4, 12, 0) is None
 
 
 class FigureCanvasGTK4(_FigureCanvasGTK, Gtk.DrawingArea):
@@ -48,7 +49,10 @@ def __init__(self, figure=None):
 
         self.set_draw_func(self._draw_func)
         self.connect('resize', self.resize_event)
-        self.connect('notify::scale-factor', self._update_device_pixel_ratio)
+        if _GTK_GE_4_12:
+            self.connect('realize', self._realize_event)
+        else:
+            self.connect('notify::scale-factor', self._update_device_pixel_ratio)
 
         click = Gtk.GestureClick()
         click.set_button(0)  # All buttons.
@@ -237,10 +241,22 @@ def _get_key(self, keyval, keycode, state):
                 and not (mod == "shift" and unikey.isprintable()))]
         return "+".join([*mods, key])
 
+    def _realize_event(self, obj):
+        surface = self.get_native().get_surface()
+        surface.connect('notify::scale', self._update_device_pixel_ratio)
+        self._update_device_pixel_ratio()
+
     def _update_device_pixel_ratio(self, *args, **kwargs):
         # We need to be careful in cases with mixed resolution displays if
         # device_pixel_ratio changes.
-        if self._set_device_pixel_ratio(self.get_scale_factor()):
+        scale = None
+        if _GTK_GE_4_12:
+            if (native := self.get_native()) is not None:
+                if (surface := native.get_surface()) is not None:
+                    scale = surface.get_scale()
+        else:
+            scale = self.get_scale_factor()
+        if scale is not None and self._set_device_pixel_ratio(scale):
             self.draw()
 
     def _draw_rubberband(self, rect):