gtk4: Fix Cairo backend on HiDPI screens

With GTK4, the Cairo context we get is always in logical pixels, and is
automatically scaled to the right size, without any worry about
blurriness. So in that case, we can ignore all scale factor changes, and
assume it's always 1.

The remaining effect of tracking scale factor changes is to trigger a
re-draw, but GTK will send a resize event to go along with it, which
will do that for us.

Fixes matplotlib#25847
Replaces matplotlib#25861

Author: QuLogic

lib/matplotlib/backends/backend_gtk4.py

@@ -34,7 +34,6 @@ class FigureCanvasGTK4(_FigureCanvasGTK, Gtk.DrawingArea):
     required_interactive_framework = "gtk4"
     supports_blit = False
     manager_class = _api.classproperty(lambda cls: FigureManagerGTK4)
-    _context_is_scaled = False
 
     def __init__(self, figure=None):
         super().__init__(figure=figure)
@@ -228,13 +227,8 @@ def _post_draw(self, widget, ctx):
 
         lw = 1
         dash = 3
-        if not self._context_is_scaled:
-            x0, y0, w, h = (dim / self.device_pixel_ratio
-                            for dim in self._rubberband_rect)
-        else:
-            x0, y0, w, h = self._rubberband_rect
-            lw *= self.device_pixel_ratio
-            dash *= self.device_pixel_ratio
+        x0, y0, w, h = (dim / self.device_pixel_ratio
+                        for dim in self._rubberband_rect)
         x1 = x0 + w
         y1 = y0 + h
 

lib/matplotlib/backends/backend_gtk4cairo.py

@@ -5,7 +5,10 @@
 
 
 class FigureCanvasGTK4Cairo(FigureCanvasCairo, FigureCanvasGTK4):
-    _context_is_scaled = True
+    def _set_device_pixel_ratio(self, ratio):
+        # Cairo in GTK4 always uses logical pixels, so we don't need to do anything for
+        # changes to the device pixel ratio.
+        return False
 
     def on_draw_event(self, widget, ctx):
         if self._idle_draw_id:
@@ -16,15 +19,11 @@ def on_draw_event(self, widget, ctx):
         with (self.toolbar._wait_cursor_for_draw_cm() if self.toolbar
               else nullcontext()):
             self._renderer.set_context(ctx)
-            scale = self.device_pixel_ratio
-            # Scale physical drawing to logical size.
-            ctx.scale(1 / scale, 1 / scale)
             allocation = self.get_allocation()
             Gtk.render_background(
                 self.get_style_context(), ctx,
                 allocation.x, allocation.y,
                 allocation.width, allocation.height)
-            self._renderer.dpi = self.figure.dpi
             self.figure.draw(self._renderer)