[mutter] backend: Get the UI scaling factor from the logical monitors

[Jonas Ådahl <jadahl src gnome org>]

commit e6eac46629335ac341e3207df8b2aac15fb9ffa7
Author: Jonas Ådahl <jadahl gmail com>
Date:   Fri Apr 7 21:54:08 2017 +0800

    backend: Get the UI scaling factor from the logical monitors
    
    Instead of looking at the GTK+ settings, check the logical monitor
    state and determine the UI scaling factor given the maximum logical
    monitor scale. This is only enabled when the monitor config manager
    feature is enabled, as only then can a scale be explicitly configured.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=777732

 src/backends/meta-backend.c |   59 +++++++++++++++++++++++++++++++++++++++++-
 1 files changed, 57 insertions(+), 2 deletions(-)
---
diff --git a/src/backends/meta-backend.c b/src/backends/meta-backend.c
index 87de4e7..61604ef 100644
--- a/src/backends/meta-backend.c
+++ b/src/backends/meta-backend.c
@@ -97,6 +97,8 @@ struct _MetaBackendPrivate
 
   MetaPointerConstraint *client_pointer_constraint;
   MetaDnd *dnd;
+
+  int ui_scaling_factor;
 };
 typedef struct _MetaBackendPrivate MetaBackendPrivate;
 
@@ -151,6 +153,25 @@ center_pointer (MetaBackend *backend)
                              primary->rect.y + primary->rect.height / 2);
 }
 
+static gboolean
+meta_backend_update_ui_scaling_factor (MetaBackend *backend)
+{
+  MetaBackendPrivate *priv = meta_backend_get_instance_private (backend);
+  int ui_scaling_factor;
+
+  ui_scaling_factor = meta_backend_get_ui_scaling_factor (backend);
+
+  if (ui_scaling_factor != priv->ui_scaling_factor)
+    {
+      priv->ui_scaling_factor = ui_scaling_factor;
+      return TRUE;
+    }
+  else
+    {
+      return FALSE;
+    }
+}
+
 void
 meta_backend_monitors_changed (MetaBackend *backend)
 {
@@ -170,6 +191,9 @@ meta_backend_monitors_changed (MetaBackend *backend)
           !meta_monitor_manager_is_headless (monitor_manager))
         center_pointer (backend);
     }
+
+  if (meta_backend_update_ui_scaling_factor (backend))
+    meta_backend_notify_ui_scaling_factor_changed (backend);
 }
 
 void
@@ -339,6 +363,8 @@ meta_backend_real_post_init (MetaBackend *backend)
 
   meta_backend_sync_screen_size (backend);
 
+  meta_backend_update_ui_scaling_factor (backend);
+
   priv->cursor_renderer = META_BACKEND_GET_CLASS (backend)->create_cursor_renderer (backend);
 
   priv->device_monitors =
@@ -983,6 +1009,7 @@ xft_dpi_changed (GtkSettings *settings,
                  GParamSpec  *pspec,
                  MetaBackend *backend)
 {
+  meta_backend_update_ui_scaling_factor (backend);
   meta_backend_notify_ui_scaling_factor_changed (backend);
 }
 
@@ -1070,13 +1097,41 @@ meta_backend_notify_keymap_layout_group_changed (MetaBackend *backend,
                  locked_group);
 }
 
+static int
+calculate_ui_scaling_factor (MetaBackend *backend)
+{
+  MetaMonitorManager *monitor_manager =
+    meta_backend_get_monitor_manager (backend);
+  GList *logical_monitors;
+  GList *l;
+  int max_scale = 1;
+
+  logical_monitors =
+    meta_monitor_manager_get_logical_monitors (monitor_manager);
+  for (l = logical_monitors; l; l = l->next)
+    {
+      MetaLogicalMonitor *logical_monitor = l->data;
+
+      max_scale = MAX (logical_monitor->scale, max_scale);
+    }
+
+  return max_scale;
+}
+
 int
 meta_backend_get_ui_scaling_factor (MetaBackend *backend)
 {
   if (meta_is_stage_views_scaled ())
-    return 1;
+    {
+      return 1;
+    }
   else
-    return meta_theme_get_window_scaling_factor ();
+    {
+      if (meta_is_monitor_config_manager_enabled ())
+        return calculate_ui_scaling_factor (backend);
+      else
+        return meta_theme_get_window_scaling_factor ();
+    }
 }
 
 void