widget: Add _gtk_widget_set_css_clip()

... and use it in GtkBox and GtkGrid.

gtk/gtkbox.c

@@ -791,6 +791,8 @@ gtk_box_size_allocate_no_center (GtkWidget     *widget,
 	  i++;
 	}
     }
+
+  _gtk_widget_set_simple_clip (widget);
 }
 
 static void
@@ -1158,6 +1160,8 @@ gtk_box_size_allocate_with_center (GtkWidget     *widget,
       child_allocation.height = center_size;
     }
   gtk_widget_size_allocate_with_baseline (priv->center->widget, &child_allocation, baseline);
+
+  _gtk_widget_set_simple_clip (widget);
 }
 
 static void

gtk/gtkgrid.c

@@ -24,6 +24,7 @@
 
 #include "gtkorientableprivate.h"
 #include "gtksizerequest.h"
+#include "gtkwidgetprivate.h"
 #include "gtkprivate.h"
 #include "gtkintl.h"
 
@@ -1677,6 +1678,8 @@ gtk_grid_size_allocate (GtkWidget     *widget,
   gtk_grid_request_position (&request, 1);
 
   gtk_grid_request_allocate_children (&request);
+
+  _gtk_widget_set_simple_clip (widget);
 }
 
 static gboolean

gtk/gtkwidget.c

@@ -15414,6 +15414,49 @@ _gtk_widget_supports_clip (GtkWidget *widget)
   return widget->priv->supports_clip;
 }
 
+static void
+union_with_clip (GtkWidget *widget,
+                 gpointer   clip)
+{
+  GtkAllocation widget_clip;
+
+  if (!gtk_widget_is_drawable (widget))
+    return;
+
+  gtk_widget_get_clip (widget, &widget_clip);
+
+  gdk_rectangle_union (&widget_clip, clip, clip);
+}
+
+/**
+ * _gtk_widget_set_simple_clip:
+ * @widget: 
+ *
+ * This is a convenience function for gtk_widget_set_clip(), if you
+ * just want to set the clip for @widget based on its allocation,
+ * CSS properties and - if the widget is a #GtkContainer - its
+ * children. gtk_widget_set_allocation() must have been called
+ * and all children must have been allocated with
+ * gtk_widget_size_allocate() before calling this function. It is
+ * therefor a good idea to call this function last in your
+ * implementation of GtkWidget::size_allocate().
+ *
+ * If your widget overdraws its contents, you cannot use this
+ * function and must call gtk_widget_set_clip() yourself.
+ **/
+void
+_gtk_widget_set_simple_clip (GtkWidget *widget)
+{
+  GtkAllocation clip;
+
+  gtk_widget_get_allocation (widget, &clip);
+
+  if (GTK_IS_CONTAINER (widget))
+    gtk_container_forall (GTK_CONTAINER (widget), union_with_clip, &clip);
+
+  gtk_widget_set_clip (widget, &clip);
+}
+
 /**
  * gtk_widget_get_allocation:
  * @widget: a #GtkWidget

gtk/gtkwidgetprivate.h

@@ -133,7 +133,8 @@ void              _gtk_widget_buildable_finish_accelerator (GtkWidget *widget,
 GtkStyle *        _gtk_widget_get_style                    (GtkWidget *widget);
 void              _gtk_widget_set_style                    (GtkWidget *widget,
                                                             GtkStyle  *style);
-gboolean          _gtk_widget_supports_clip                (GtkWidget           *widget);
+gboolean          _gtk_widget_supports_clip                (GtkWidget *widget);
+void              _gtk_widget_set_simple_clip              (GtkWidget *widget);
 
 typedef gboolean (*GtkCapturedEventHandler) (GtkWidget *widget, GdkEvent *event);