widget: Add a system_setting_changed vfunc

This gives us a hook to walk the widget tree whenever a global
setting changes and do per-widget invalidations. This will
replace gtk_style_context_reset_widgets().

docs/reference/gtk/gtk4-sections.txt

@@ -2312,6 +2312,7 @@ GtkSettingsValue
 gtk_settings_get_default
 gtk_settings_get_for_display
 gtk_settings_reset_property
+GtkSystemSetting
 <SUBSECTION Standard>
 GtkSettingsClass
 GTK_IS_SETTINGS

gtk/gtkenums.h

@@ -1088,4 +1088,33 @@ typedef enum {
   GTK_CONSTRAINT_VFL_PARSER_ERROR_INVALID_RELATION
 } GtkConstraintVflParserError;
 
+/**
+ * GtkSystemSetting:
+ * @GTK_SYSTEM_SETTING_DPI: the #GtkSettings:gtk-xft-dpi setting has changed
+ * @GTK_SYSTEM_SETTING_FONT_NAME: The #GtkSettings:gtk-font-name setting has changed
+ * @GTK_SYSTEM_SETTING_FONT_CONFIG: The font configuration has changed in a way that
+ *     requires text to be redrawn. This can be any of the
+ *     #GtkSettings:gtk-xft-antialias, #GtkSettings:gtk-xft-hinting,
+ *     #GtkSettings:gtk-xft-hintstyle, #GtkSettings:gtk-xft-rgba or
+ *     #GtkSettings:gtk-fontconfig-timestamp settings
+ * @GTK_SYSTEM_SETTING_DISPLAY: The display has changed
+ * @GTK_SYSTEM_SETTING_ICON_THEME: The icon theme has changed in a way that requires
+ *     icons to be looked up again
+ *
+ * Values that can be passed to the GtkWidgetClass.system_setting_changed
+ * vfunc to indicate that a system setting has changed and widgets may
+ * need to drop caches, or react otherwise.
+ *
+ * Most of the values correspond to #GtkSettings properties.
+ *
+ * More values may be added over time.
+ */
+typedef enum {
+  GTK_SYSTEM_SETTING_DPI,
+  GTK_SYSTEM_SETTING_FONT_NAME,
+  GTK_SYSTEM_SETTING_FONT_CONFIG,
+  GTK_SYSTEM_SETTING_DISPLAY,
+  GTK_SYSTEM_SETTING_ICON_THEME
+} GtkSystemSetting;
+
 #endif /* __GTK_ENUMS_H__ */

gtk/gtkwidget.c

@@ -592,6 +592,8 @@ static gboolean gtk_widget_real_query_tooltip    (GtkWidget         *widget,
 						  GtkTooltip        *tooltip);
 static void     gtk_widget_real_css_changed      (GtkWidget         *widget,
                                                   GtkCssStyleChange *change);
+static void     gtk_widget_real_system_setting_changed (GtkWidget         *widget,
+                                                        GtkSystemSetting   setting);
 
 static void             gtk_widget_real_set_focus_child         (GtkWidget        *widget,
                                                                  GtkWidget        *child);
@@ -911,6 +913,7 @@ gtk_widget_class_init (GtkWidgetClass *klass)
   klass->keynav_failed = gtk_widget_real_keynav_failed;
   klass->query_tooltip = gtk_widget_real_query_tooltip;
   klass->css_changed = gtk_widget_real_css_changed;
+  klass->system_setting_changed = gtk_widget_real_system_setting_changed;
 
   /* Accessibility support */
   klass->priv->accessible_type = GTK_TYPE_ACCESSIBLE;
@@ -4865,6 +4868,20 @@ gtk_widget_real_css_changed (GtkWidget         *widget,
     }
 }
 
+static void
+gtk_widget_real_system_setting_changed (GtkWidget        *widget,
+                                        GtkSystemSetting  setting)
+{
+  GtkWidget *child;
+
+  for (child = _gtk_widget_get_first_child (widget);
+       child != NULL;
+       child = _gtk_widget_get_next_sibling (child))
+    {
+      gtk_widget_system_setting_changed (child, setting);
+    }
+}
+
 static gboolean
 direction_is_forward (GtkDirectionType direction)
 {
@@ -10627,6 +10644,32 @@ gtk_widget_css_changed (GtkWidget         *widget,
   GTK_WIDGET_GET_CLASS (widget)->css_changed (widget, change);
 }
 
+void
+gtk_widget_system_setting_changed (GtkWidget        *widget,
+                                   GtkSystemSetting  setting)
+{
+  GTK_WIDGET_GET_CLASS (widget)->system_setting_changed (widget, setting);
+}
+
+void
+gtk_system_setting_changed (GdkDisplay       *display,
+                            GtkSystemSetting  setting)
+{
+  GList *list, *toplevels;
+
+  toplevels = gtk_window_list_toplevels ();
+  g_list_foreach (toplevels, (GFunc) g_object_ref, NULL);
+
+  for (list = toplevels; list; list = list->next)
+    {
+      if (gtk_widget_get_display (list->data) == display)
+        gtk_widget_system_setting_changed (list->data, setting);
+      g_object_unref (list->data);
+    }
+
+  g_list_free (toplevels);
+}
+
 GtkCssNode *
 gtk_widget_get_css_node (GtkWidget *widget)
 {

gtk/gtkwidget.h

@@ -201,6 +201,7 @@ struct _GtkWidget
  *   should then discard their caches that depend on CSS and queue resizes or
  *   redraws accordingly. The default implementation will take care of this for
  *   all the default CSS properties, so implementations must chain up.
+ * @system_setting_changed: Emitted when a system setting was changed. Must chain up.
  * @snapshot: Vfunc for gtk_widget_snapshot().
  * @contains: Vfunc for gtk_widget_contains().
  */
@@ -277,6 +278,9 @@ struct _GtkWidgetClass
   void         (* css_changed)                 (GtkWidget            *widget,
                                                 GtkCssStyleChange    *change);
 
+  void         (* system_setting_changed)      (GtkWidget            *widget,
+                                                GtkSystemSetting      settings);
+
   void         (* snapshot)                    (GtkWidget            *widget,
                                                 GtkSnapshot          *snapshot);
 

gtk/gtkwidgetprivate.h

@@ -282,6 +282,10 @@ gboolean          _gtk_widget_captured_event               (GtkWidget *widget,
 
 void              gtk_widget_css_changed                   (GtkWidget           *widget,
                                                             GtkCssStyleChange   *change);
+void              gtk_widget_system_setting_changed        (GtkWidget           *widget,
+                                                            GtkSystemSetting     setting);
+void              gtk_system_setting_changed               (GdkDisplay          *display,
+                                                            GtkSystemSetting     setting);
 
 void              _gtk_widget_update_parent_muxer          (GtkWidget    *widget);
 GtkActionMuxer *  _gtk_widget_get_action_muxer             (GtkWidget    *widget,