Backport PR matplotlib#30344: Support fractional HiDPI in GTK4 backend

Author: tacaswell

galleries/examples/user_interfaces/embedding_in_gtk4_panzoom_sgskip.py

@@ -44,10 +44,9 @@ def on_activate(app):
     toolbar = NavigationToolbar(canvas)
     vbox.append(toolbar)
 
-    win.show()
+    win.present()
 
 
-app = Gtk.Application(
-    application_id='org.matplotlib.examples.EmbeddingInGTK4PanZoom')
+app = Gtk.Application(application_id='org.matplotlib.examples.EmbeddingInGTK4PanZoom')
 app.connect('activate', on_activate)
 app.run(None)

galleries/examples/user_interfaces/embedding_in_gtk4_sgskip.py

@@ -39,7 +39,7 @@ def on_activate(app):
     canvas.set_size_request(800, 600)
     sw.set_child(canvas)
 
-    win.show()
+    win.present()
 
 
 app = Gtk.Application(application_id='org.matplotlib.examples.EmbeddingInGTK4')

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 +241,20 @@ 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()):
+        if _GTK_GE_4_12:
+            scale = self.get_native().get_surface().get_scale()
+        else:
+            scale = self.get_scale_factor()
+        assert scale is not None
+        if self._set_device_pixel_ratio(scale):
             self.draw()
 
     def _draw_rubberband(self, rect):