core/window: Track monitor with the highest scale value

This is used for Wayland fractional scale pixel alignment.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2726>

src/backends/meta-monitor-manager-private.h

@@ -293,6 +293,9 @@ MetaLogicalMonitor *meta_monitor_manager_get_logical_monitor_at (MetaMonitorMana
 MetaLogicalMonitor *meta_monitor_manager_get_logical_monitor_from_rect (MetaMonitorManager *manager,
                                                                         MetaRectangle      *rect);
 
+MetaLogicalMonitor *meta_monitor_manager_get_highest_scale_monitor_from_rect (MetaMonitorManager *manager,
+                                                                              MetaRectangle      *rect);
+
 MetaLogicalMonitor *meta_monitor_manager_get_logical_monitor_neighbor (MetaMonitorManager  *manager,
                                                                        MetaLogicalMonitor  *logical_monitor,
                                                                        MetaDisplayDirection direction);

src/backends/meta-monitor-manager.c

@@ -3399,6 +3399,47 @@ meta_monitor_manager_get_logical_monitor_from_rect (MetaMonitorManager *manager,
   return best_logical_monitor;
 }
 
+/**
+ * meta_monitor_manager_get_highest_scale_from_rect:
+ * @manager: A #MetaMonitorManager object
+ * @rect: The rectangle
+ *
+ * Finds the #MetaLogicalMonitor with the highest scale intersecting @rect.
+ *
+ * Returns: (transfer none) (nullable): the #MetaLogicalMonitor with the
+ *          highest scale intersecting with @rect, or %NULL if none.
+ */
+MetaLogicalMonitor *
+meta_monitor_manager_get_highest_scale_monitor_from_rect (MetaMonitorManager *manager,
+                                                          MetaRectangle      *rect)
+{
+  MetaLogicalMonitor *best_logical_monitor = NULL;
+  GList *l;
+  float best_scale = 0.0;
+
+  for (l = manager->logical_monitors; l; l = l->next)
+    {
+      MetaLogicalMonitor *logical_monitor = l->data;
+      MetaRectangle intersection;
+      float scale;
+
+      if (!meta_rectangle_intersect (&logical_monitor->rect,
+                                     rect,
+                                     &intersection))
+        continue;
+
+      scale = meta_logical_monitor_get_scale (logical_monitor);
+
+      if (scale > best_scale)
+        {
+          best_scale = scale;
+          best_logical_monitor = logical_monitor;
+        }
+    }
+
+  return best_logical_monitor;
+}
+
 MetaLogicalMonitor *
 meta_monitor_manager_get_logical_monitor_neighbor (MetaMonitorManager  *manager,
                                                    MetaLogicalMonitor  *logical_monitor,

src/core/window-private.h

@@ -168,6 +168,7 @@ struct _MetaWindow
   uint64_t id;
   guint64 stamp;
   MetaLogicalMonitor *monitor;
+  MetaLogicalMonitor *highest_scale_monitor;
   MetaWorkspace *workspace;
   MetaWindowClientType client_type;
   Window xwindow;
@@ -828,6 +829,7 @@ MetaLogicalMonitor * meta_window_find_monitor_from_frame_rect (MetaWindow *windo
 MetaLogicalMonitor * meta_window_find_monitor_from_id (MetaWindow *window);
 
 MetaLogicalMonitor * meta_window_get_main_logical_monitor (MetaWindow *window);
+MetaLogicalMonitor * meta_window_get_highest_scale_monitor (MetaWindow *window);
 void meta_window_update_monitor (MetaWindow                   *window,
                                  MetaWindowUpdateMonitorFlags  flags);
 

src/core/window.c

@@ -221,6 +221,7 @@ enum
   SIZE_CHANGED,
   POSITION_CHANGED,
   SHOWN,
+  HIGHEST_SCALE_MONITOR_CHANGED,
 
   LAST_SIGNAL
 };
@@ -755,6 +756,21 @@ meta_window_class_init (MetaWindowClass *klass)
                   0,
                   NULL, NULL, NULL,
                   G_TYPE_NONE, 0);
+
+  /**
+   * MetaWindow::highest-scale-monitor-changed:
+   * @window: a #MetaWindow
+   *
+   * This is emitted when the monitor with the highest scale
+   * intersecting the window changes.
+   */
+  window_signals[HIGHEST_SCALE_MONITOR_CHANGED] =
+    g_signal_new ("highest-scale-monitor-changed",
+                  G_TYPE_FROM_CLASS (object_class),
+                  G_SIGNAL_RUN_LAST,
+                  0,
+                  NULL, NULL, NULL,
+                  G_TYPE_NONE, 0);
 }
 
 static void
@@ -1007,6 +1023,19 @@ meta_window_find_monitor_from_frame_rect (MetaWindow *window)
                                                              &window_rect);
 }
 
+static MetaLogicalMonitor *
+meta_window_find_highest_scale_monitor_from_frame_rect (MetaWindow *window)
+{
+  MetaBackend *backend = backend_from_window (window);
+  MetaMonitorManager *monitor_manager =
+    meta_backend_get_monitor_manager (backend);
+  MetaRectangle window_rect;
+
+  meta_window_get_frame_rect (window, &window_rect);
+  return meta_monitor_manager_get_highest_scale_monitor_from_rect (monitor_manager,
+                                                                   &window_rect);
+}
+
 static void
 meta_window_manage (MetaWindow *window)
 {
@@ -1152,6 +1181,9 @@ meta_window_constructed (GObject *object)
   else
     window->monitor = meta_backend_get_current_logical_monitor (backend);
 
+  window->highest_scale_monitor =
+    meta_window_find_highest_scale_monitor_from_frame_rect (window);
+
   if (window->monitor)
     window->preferred_output_winsys_id = window->monitor->winsys_id;
   else
@@ -3573,6 +3605,12 @@ meta_window_get_main_logical_monitor (MetaWindow *window)
   return window->monitor;
 }
 
+MetaLogicalMonitor *
+meta_window_get_highest_scale_monitor (MetaWindow *window)
+{
+  return window->highest_scale_monitor;
+}
+
 static MetaLogicalMonitor *
 find_monitor_by_winsys_id (MetaWindow *window,
                            uint64_t    winsys_id)
@@ -3684,7 +3722,7 @@ meta_window_update_monitor (MetaWindow                   *window,
                             MetaWindowUpdateMonitorFlags  flags)
 {
   MetaWorkspaceManager *workspace_manager = window->display->workspace_manager;
-  const MetaLogicalMonitor *old;
+  const MetaLogicalMonitor *old, *old_highest_scale;
 
   old = window->monitor;
   META_WINDOW_GET_CLASS (window)->update_main_monitor (window, flags);
@@ -3715,6 +3753,12 @@ meta_window_update_monitor (MetaWindow                   *window,
 
       meta_display_queue_check_fullscreen (window->display);
     }
+
+  old_highest_scale = window->highest_scale_monitor;
+  window->highest_scale_monitor =
+    meta_window_find_highest_scale_monitor_from_frame_rect (window);
+  if (old_highest_scale != window->highest_scale_monitor)
+    g_signal_emit (window, window_signals[HIGHEST_SCALE_MONITOR_CHANGED], 0);
 }
 
 void