monitor-manager: Check if all monitor scales are matching in global mode

When global scaling is set we need to ensure that all the requested scale
configurations are matching, otherwise we'd end up in a mixed setup that
we don't support in this scenario.

Fixes https://gitlab.gnome.org/GNOME/mutter/issues/407

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/336>

src/backends/meta-monitor-manager.c

@@ -125,6 +125,10 @@ meta_monitor_manager_get_active_monitor (MetaMonitorManager *manager);
 static void
 meta_monitor_manager_real_read_current_state (MetaMonitorManager *manager);
 
+static gboolean
+is_global_scale_matching_in_config (MetaMonitorsConfig *config,
+                                    float               scale);
+
 MetaBackend *
 meta_monitor_manager_get_backend (MetaMonitorManager *manager)
 {
@@ -1761,6 +1765,43 @@ meta_monitor_manager_is_scale_supported (MetaMonitorManager          *manager,
   return FALSE;
 }
 
+static gboolean
+is_global_scale_matching_in_config (MetaMonitorsConfig *config,
+                                    float               scale)
+{
+  GList *l;
+
+  for (l = config->logical_monitor_configs; l; l = l->next)
+    {
+      MetaLogicalMonitorConfig *logical_monitor_config = l->data;
+
+      if (!G_APPROX_VALUE (logical_monitor_config->scale, scale, FLT_EPSILON))
+        return FALSE;
+    }
+
+  return TRUE;
+}
+
+static gboolean
+meta_monitor_manager_is_scale_supported_for_config (MetaMonitorManager *manager,
+                                                    MetaMonitorsConfig *config,
+                                                    MetaMonitor        *monitor,
+                                                    MetaMonitorMode    *monitor_mode,
+                                                    float               scale)
+{
+  if (meta_monitor_manager_is_scale_supported (manager, config->layout_mode,
+                                               monitor, monitor_mode, scale))
+    {
+      if (meta_monitor_manager_get_capabilities (manager) &
+          META_MONITOR_MANAGER_CAPABILITY_GLOBAL_SCALE_REQUIRED)
+        return is_global_scale_matching_in_config (config, scale);
+
+      return TRUE;
+    }
+
+  return FALSE;
+}
+
 static gboolean
 meta_monitor_manager_is_config_applicable (MetaMonitorManager *manager,
                                            MetaMonitorsConfig *config,
@@ -1799,11 +1840,11 @@ meta_monitor_manager_is_config_applicable (MetaMonitorManager *manager,
               return FALSE;
             }
 
-          if (!meta_monitor_manager_is_scale_supported (manager,
+          if (!meta_monitor_manager_is_scale_supported_for_config (manager,
-                                                        config->layout_mode,
+                                                                   config,
-                                                        monitor,
+                                                                   monitor,
-                                                        monitor_mode,
+                                                                   monitor_mode,
-                                                        scale))
+                                                                   scale))
             {
               g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
                            "Scale not supported by backend");