x11-display: Expose UI scaling factor via D-Bus

This replaces the `legacy-ui-scaling-factor` entry in
`org.gnome.Mutter.DisplayConfig`, with the motivation being to no longer
expose X11 specific state via the monitor configuration API.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3567>

data/dbus-interfaces/org.gnome.Mutter.DisplayConfig.xml

@@ -426,10 +426,6 @@
 				       always use the same scale. Absence of
 				       this means logical monitor scales can
 				       differ.
-	* "legacy-ui-scaling-factor" (i): The legacy scaling factor traditionally
-				          used to scale X11 clients (commonly
-					  communicated via the
-					  Gdk/WindowScalingFactor XSetting entry).
     -->
     <method name="GetCurrentState">
       <arg name="serial" direction="out" type="u" />

data/dbus-interfaces/org.gnome.Mutter.X11.xml

@@ -0,0 +1,8 @@
+<!DOCTYPE node PUBLIC
+'-//freedesktop//DTD D-BUS Object Introspection 1.0//EN'
+'http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd'>
+<node>
+  <interface name="org.gnome.Mutter.X11">
+    <property name="UiScalingFactor" type="i" access="readwrite" />
+  </interface>
+</node>

src/backends/meta-monitor-manager.c

@@ -1948,14 +1948,12 @@ meta_monitor_manager_handle_get_current_state (MetaDBusDisplayConfig *skeleton,
                                                GDBusMethodInvocation *invocation,
                                                MetaMonitorManager    *manager)
 {
-  MetaSettings *settings = meta_backend_get_settings (manager->backend);
   GVariantBuilder monitors_builder;
   GVariantBuilder logical_monitors_builder;
   GVariantBuilder properties_builder;
   GList *l;
   int i;
   MetaMonitorManagerCapability capabilities;
-  int ui_scaling_factor;
   int max_screen_width, max_screen_height;
 
   g_variant_builder_init (&monitors_builder,
@@ -2158,11 +2156,6 @@ meta_monitor_manager_handle_get_current_state (MetaDBusDisplayConfig *skeleton,
                              g_variant_new_boolean (TRUE));
     }
 
-  ui_scaling_factor = meta_settings_get_ui_scaling_factor (settings);
-  g_variant_builder_add (&properties_builder, "{sv}",
-                         "legacy-ui-scaling-factor",
-                         g_variant_new_int32 (ui_scaling_factor));
-
   if (meta_monitor_manager_get_max_screen_size (manager,
                                                 &max_screen_width,
                                                 &max_screen_height))

src/meson.build

@@ -968,6 +968,11 @@ dbus_interfaces = [
     'interface': 'org.gnome.Mutter.DebugControl.xml',
     'prefix': 'org.gnome.Mutter',
   },
+  {
+    'name': 'meta-dbus-x11',
+    'interface': 'org.gnome.Mutter.X11.xml',
+    'prefix': 'org.gnome.Mutter',
+  },
 ]
 
 if have_profiler

src/x11/meta-x11-display.c

@@ -71,7 +71,7 @@
 #include "wayland/meta-xwayland-private.h"
 #endif
 
-G_DEFINE_TYPE (MetaX11Display, meta_x11_display, G_TYPE_OBJECT)
+#include "meta-dbus-x11.h"
 
 static GQuark quark_x11_display_logical_monitor_data = 0;
 
@@ -90,6 +90,14 @@ typedef struct _MetaX11DisplayLogicalMonitorData
   int xinerama_index;
 } MetaX11DisplayLogicalMonitorData;
 
+typedef struct _MetaX11DisplayPrivate
+{
+  MetaDBusX11 *dbus_api;
+  guint dbus_name_id;
+} MetaX11DisplayPrivate;
+
+G_DEFINE_TYPE_WITH_PRIVATE (MetaX11Display, meta_x11_display, G_TYPE_OBJECT)
+
 static char *get_screen_name (Display *xdisplay,
                               int      number);
 
@@ -152,13 +160,46 @@ meta_x11_event_filter_free (MetaX11EventFilter *filter)
   g_free (filter);
 }
 
+static void
+on_bus_acquired (GDBusConnection *connection,
+                 const char      *name,
+                 gpointer         user_data)
+{
+  MetaX11Display *x11_display = user_data;
+  MetaX11DisplayPrivate *priv =
+    meta_x11_display_get_instance_private (x11_display);
+
+  g_dbus_interface_skeleton_export (G_DBUS_INTERFACE_SKELETON (priv->dbus_api),
+                                    connection,
+                                    "/org/gnome/Mutter/X11",
+                                    NULL);
+}
+
+static void
+update_ui_scaling_factor (MetaX11Display *x11_display)
+{
+  MetaX11DisplayPrivate *priv =
+    meta_x11_display_get_instance_private (x11_display);
+  MetaBackend *backend = backend_from_x11_display (x11_display);
+  MetaSettings *settings = meta_backend_get_settings (backend);
+  int ui_scaling_factor;
+
+  ui_scaling_factor = meta_settings_get_ui_scaling_factor (settings);
+  meta_dbus_x11_set_ui_scaling_factor (priv->dbus_api, ui_scaling_factor);
+}
+
 static void
 meta_x11_display_dispose (GObject *object)
 {
   MetaX11Display *x11_display = META_X11_DISPLAY (object);
+  MetaX11DisplayPrivate *priv =
+    meta_x11_display_get_instance_private (x11_display);
 
   x11_display->closing = TRUE;
 
+  g_clear_handle_id (&priv->dbus_name_id, g_bus_unown_name);
+  g_clear_object (&priv->dbus_api);
+
   g_clear_pointer (&x11_display->alarm_filters, g_ptr_array_unref);
 
   g_clear_list (&x11_display->event_funcs,
@@ -1221,6 +1262,23 @@ meta_x11_display_init_frames_client (MetaX11Display *x11_display)
                            on_frames_client_died, x11_display);
 }
 
+static void
+initialize_dbus_interface (MetaX11Display *x11_display)
+{
+  MetaX11DisplayPrivate *priv =
+    meta_x11_display_get_instance_private (x11_display);
+
+  priv->dbus_api = meta_dbus_x11_skeleton_new ();
+  priv->dbus_name_id =
+    g_bus_own_name (G_BUS_TYPE_SESSION,
+                    "org.gnome.Mutter.X11",
+                    G_BUS_NAME_OWNER_FLAGS_NONE,
+                    on_bus_acquired,
+                    NULL, NULL,
+                    x11_display, NULL);
+  update_ui_scaling_factor (x11_display);
+}
+
 /**
  * meta_x11_display_new:
  *
@@ -1317,6 +1375,8 @@ meta_x11_display_new (MetaDisplay  *display,
   x11_display = g_object_new (META_TYPE_X11_DISPLAY, NULL);
   x11_display->display = display;
 
+  initialize_dbus_interface (x11_display);
+
   /* here we use XDisplayName which is what the user
    * probably put in, vs. DisplayString(display) which is
    * canonicalized by XOpenDisplay()
@@ -1980,6 +2040,8 @@ on_monitors_changed_internal (MetaMonitorManager *monitor_manager,
     }
 
   x11_display->has_xinerama_indices = FALSE;
+
+  update_ui_scaling_factor (x11_display);
 }
 
 static Bool