backends: Check both input settings and mapper for tablet monitors

The upper layers (OSDs basically) want to know the monitor that a
tablet is currently assigned to, not the monitor just as configured
through settings.

This broke proper OSD positioning for display-attached tablets since
commit 87858a4e01, as the MetaInputMapper kicks in precisely when
there is no configured monitor for the given device.

Consulting both about the assigned output will make OSDs pop up
again in the right place.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/971

src/backends/meta-input-mapper-private.h

@@ -42,5 +42,8 @@ ClutterInputDevice *
 meta_input_mapper_get_logical_monitor_device (MetaInputMapper        *mapper,
                                               MetaLogicalMonitor     *logical_monitor,
                                               ClutterInputDeviceType  device_type);
+MetaLogicalMonitor *
+meta_input_mapper_get_device_logical_monitor (MetaInputMapper *mapper,
+                                              ClutterInputDevice *device);
 
 #endif /* META_INPUT_MAPPER_H */

src/backends/meta-input-mapper.c

@@ -675,3 +675,29 @@ meta_input_mapper_get_logical_monitor_device (MetaInputMapper        *mapper,
 
   return NULL;
 }
+
+MetaLogicalMonitor *
+meta_input_mapper_get_device_logical_monitor (MetaInputMapper    *mapper,
+                                              ClutterInputDevice *device)
+{
+  MetaMapperOutputInfo *output;
+  MetaLogicalMonitor *logical_monitor;
+  GHashTableIter iter;
+  GList *l;
+
+  g_hash_table_iter_init (&iter, mapper->output_devices);
+
+  while (g_hash_table_iter_next (&iter, (gpointer *) &logical_monitor,
+                                 (gpointer *) &output))
+    {
+      for (l = output->input_devices; l; l = l->next)
+        {
+          MetaMapperInputInfo *input = l->data;
+
+          if (input->device == device)
+            return logical_monitor;
+        }
+    }
+
+  return NULL;
+}

src/backends/meta-input-settings.c

@@ -2064,6 +2064,44 @@ meta_input_settings_get_tablet_settings (MetaInputSettings  *settings,
   return info ? g_object_ref (info->settings) : NULL;
 }
 
+static ClutterInputDevice *
+find_grouped_pen (MetaInputSettings  *settings,
+                  ClutterInputDevice *device)
+{
+  MetaInputSettingsPrivate *priv;
+  GList *l, *devices;
+  ClutterInputDeviceType device_type;
+  ClutterInputDevice *pen = NULL;
+
+  device_type = clutter_input_device_get_device_type (device);
+
+  if (device_type == CLUTTER_TABLET_DEVICE ||
+      device_type == CLUTTER_PEN_DEVICE)
+    return device;
+
+  priv = meta_input_settings_get_instance_private (settings);
+  devices = clutter_seat_list_devices (priv->seat);
+
+  for (l = devices; l; l = l->next)
+    {
+      ClutterInputDevice *other_device = l->data;
+
+      device_type = clutter_input_device_get_device_type (other_device);
+
+      if ((device_type == CLUTTER_TABLET_DEVICE ||
+           device_type == CLUTTER_PEN_DEVICE) &&
+          clutter_input_device_is_grouped (device, other_device))
+        {
+          pen = other_device;
+          break;
+        }
+    }
+
+  g_list_free (devices);
+
+  return pen;
+}
+
 MetaLogicalMonitor *
 meta_input_settings_get_tablet_logical_monitor (MetaInputSettings  *settings,
                                                 ClutterInputDevice *device)
@@ -2075,13 +2113,27 @@ meta_input_settings_get_tablet_logical_monitor (MetaInputSettings  *settings,
   g_return_val_if_fail (META_IS_INPUT_SETTINGS (settings), NULL);
   g_return_val_if_fail (CLUTTER_IS_INPUT_DEVICE (device), NULL);
 
+  if (clutter_input_device_get_device_type (device) == CLUTTER_PAD_DEVICE)
+    {
+      device = find_grouped_pen (settings, device);
+      if (!device)
+        return NULL;
+    }
+
   priv = meta_input_settings_get_instance_private (settings);
   info = g_hash_table_lookup (priv->mappable_devices, device);
   if (!info)
     return NULL;
 
+  logical_monitor =
+    meta_input_mapper_get_device_logical_monitor (priv->input_mapper, device);
+
+  if (!logical_monitor)
+    {
       meta_input_settings_find_monitor (settings, info->settings, device,
                                         NULL, &logical_monitor);
+    }
+
   return logical_monitor;
 }