st: Report correct paint volumes during transitions

StWidget reports a paint volume large enough to paint the current
theme node. As CSS transitions also paint the previous theme node,
the reported paint volume may be incorrect, resulting in screen
artifacts when painting outside the reported volume.

Add st_theme_node_transition_get_paint_box() to calculate an allocation
large enough to paint both theme nodes, and use it to report the correct
paint volume during transitions.

https://bugzilla.gnome.org/show_bug.cgi?id=640085

src/st/st-theme-node-transition.c

@@ -182,6 +182,22 @@ st_theme_node_transition_update (StThemeNodeTransition *transition,
 static void
 calculate_offscreen_box (StThemeNodeTransition *transition,
                          const ClutterActorBox *allocation)
+{
+  ClutterActorBox paint_box;
+
+  st_theme_node_transition_get_paint_box (transition,
+                                          allocation,
+                                          &paint_box);
+  transition->priv->offscreen_box.x1 = paint_box.x1 - allocation->x1;
+  transition->priv->offscreen_box.y1 = paint_box.y1 - allocation->y1;
+  transition->priv->offscreen_box.x2 = paint_box.x2 - allocation->x1;
+  transition->priv->offscreen_box.y2 = paint_box.y2 - allocation->y1;
+}
+
+void
+st_theme_node_transition_get_paint_box (StThemeNodeTransition *transition,
+                                        const ClutterActorBox *allocation,
+                                        ClutterActorBox       *paint_box)
 {
   StThemeNodeTransitionPrivate *priv = transition->priv;
   ClutterActorBox old_node_box, new_node_box;
@@ -194,14 +210,10 @@ calculate_offscreen_box (StThemeNodeTransition *transition,
                                allocation,
                                &new_node_box);
 
-  priv->offscreen_box.x1 = MIN (old_node_box.x1, new_node_box.x1)
+  paint_box->x1 = MIN (old_node_box.x1, new_node_box.x1);
-                           - allocation->x1;
+  paint_box->y1 = MIN (old_node_box.y1, new_node_box.y1);
-  priv->offscreen_box.y1 = MIN (old_node_box.y1, new_node_box.y1)
+  paint_box->x2 = MAX (old_node_box.x2, new_node_box.x2);
-                           - allocation->y1;
+  paint_box->y2 = MAX (old_node_box.y2, new_node_box.y2);
-  priv->offscreen_box.x2 = MAX (old_node_box.x2, new_node_box.x2)
-                           - allocation->x1;
-  priv->offscreen_box.y2 = MAX (old_node_box.y2, new_node_box.y2)
-                           - allocation->y1;
 }
 
 static gboolean

src/st/st-theme-node-transition.h

@@ -65,6 +65,10 @@ void  st_theme_node_transition_paint    (StThemeNodeTransition *transition,
                                          ClutterActorBox       *allocation,
                                          guint8                 paint_opacity);
 
+void  st_theme_node_transition_get_paint_box (StThemeNodeTransition *transition,
+                                              const ClutterActorBox *allocation,
+                                              ClutterActorBox       *paint_box);
+
 G_END_DECLS
 
 #endif

src/st/st-widget.c

@@ -686,15 +686,23 @@ st_widget_get_paint_volume (ClutterActor *self, ClutterPaintVolume *volume)
 {
   ClutterActorBox paint_box, alloc_box;
   StThemeNode *theme_node;
+  StWidgetPrivate *priv;
   ClutterVertex origin;
 
   /* Setting the paint volume does not make sense when we don't have any allocation */
   if (!clutter_actor_has_allocation (self))
     return FALSE;
 
+  priv = ST_WIDGET(self)->priv;
+
   theme_node = st_widget_get_theme_node (ST_WIDGET(self));
   clutter_actor_get_allocation_box (self, &alloc_box);
-  st_theme_node_get_paint_box (theme_node, &alloc_box, &paint_box);
+
+  if (priv->transition_animation)
+    st_theme_node_transition_get_paint_box (priv->transition_animation,
+                                            &alloc_box, &paint_box);
+  else
+    st_theme_node_get_paint_box (theme_node, &alloc_box, &paint_box);
 
   origin.x = paint_box.x1 - alloc_box.x1;
   origin.y = paint_box.y1 - alloc_box.y1;