diff --git a/gtk/gtkapplicationwindow.c b/gtk/gtkapplicationwindow.c
index 271054b109..541baf6a03 100644
--- a/gtk/gtkapplicationwindow.c
+++ b/gtk/gtkapplicationwindow.c
@@ -703,13 +703,17 @@ gtk_application_window_real_size_allocate (GtkWidget     *widget,
 
   if (window->priv->menubar != NULL)
     {
-      GtkAllocation menubar_allocation = *allocation;
+      GtkAllocation menubar_allocation;
+      GtkAllocation child_allocation;
       gint menubar_height;
       GtkWidget *child;
 
-      _gtk_window_set_allocation (GTK_WINDOW (widget), allocation);
+      _gtk_window_set_allocation (GTK_WINDOW (widget), allocation, &child_allocation);
+      menubar_allocation = child_allocation;
 
-      gtk_widget_get_preferred_height_for_width (window->priv->menubar, allocation->width, &menubar_height, NULL);
+      gtk_widget_get_preferred_height_for_width (window->priv->menubar,
+                                                 menubar_allocation.width,
+                                                 &menubar_height, NULL);
 
       menubar_allocation.height = menubar_height;
       gtk_widget_size_allocate (window->priv->menubar, &menubar_allocation);
@@ -717,7 +721,6 @@ gtk_application_window_real_size_allocate (GtkWidget     *widget,
       child = gtk_bin_get_child (GTK_BIN (window));
       if (child != NULL && gtk_widget_get_visible (child))
         {
-          GtkAllocation child_allocation = *allocation;
           gint border_width;
 
           border_width = gtk_container_get_border_width (GTK_CONTAINER (window));
diff --git a/gtk/gtkwindow.c b/gtk/gtkwindow.c
index fe24363fd1..9f9a4d59c2 100644
--- a/gtk/gtkwindow.c
+++ b/gtk/gtkwindow.c
@@ -5504,7 +5504,9 @@ set_grip_position (GtkWindow *window)
 
 /* _gtk_window_set_allocation:
  * @window: a #GtkWindow
- * @allocation: the new allocation
+ * @allocation: the original allocation for the window
+ * @allocation_out: @allocation taking decorations into
+ * consideration
  *
  * This function is like gtk_widget_set_allocation()
  * but does the necessary extra work to update
@@ -5513,13 +5515,22 @@ set_grip_position (GtkWindow *window)
  * Call this instead of gtk_widget_set_allocation()
  * when overriding ::size_allocate in a GtkWindow
  * subclass without chaining up.
+ *
+ * The @allocation parameter will be adjusted to
+ * reflect any internal decorations that the window
+ * may have. That revised allocation will then be
+ * returned in the @allocation_out parameter.
  */
 void
-_gtk_window_set_allocation (GtkWindow     *window,
-                            GtkAllocation *allocation)
+_gtk_window_set_allocation (GtkWindow           *window,
+                            const GtkAllocation *allocation,
+                            GtkAllocation       *allocation_out)
 {
   GtkWidget *widget = (GtkWidget *)window;
 
+  g_assert (allocation != NULL);
+  g_assert (allocation_out != NULL);
+
   gtk_widget_set_allocation (widget, allocation);
 
   if (gtk_widget_get_realized (widget))
@@ -5539,6 +5550,8 @@ _gtk_window_set_allocation (GtkWindow     *window,
           set_grip_position (window);
         }
     }
+
+  *allocation_out = *allocation;
 }
 
 static void
@@ -5550,16 +5563,16 @@ gtk_window_size_allocate (GtkWidget     *widget,
   GtkWidget *child;
   guint border_width;
 
-  _gtk_window_set_allocation (window, allocation);
+  _gtk_window_set_allocation (window, allocation, &child_allocation);
 
   child = gtk_bin_get_child (&(window->bin));
   if (child && gtk_widget_get_visible (child))
     {
       border_width = gtk_container_get_border_width (GTK_CONTAINER (window));
-      child_allocation.x = border_width;
-      child_allocation.y = border_width;
-      child_allocation.width = MAX (1, allocation->width - border_width * 2);
-      child_allocation.height = MAX (1, allocation->height - border_width * 2);
+      child_allocation.x += border_width;
+      child_allocation.y += border_width;
+      child_allocation.width = MAX (1, child_allocation.width - border_width * 2);
+      child_allocation.height = MAX (1, child_allocation.height - border_width * 2);
 
       gtk_widget_size_allocate (child, &child_allocation);
     }
diff --git a/gtk/gtkwindowprivate.h b/gtk/gtkwindowprivate.h
index 22f8d57705..8891ca4c4f 100644
--- a/gtk/gtkwindowprivate.h
+++ b/gtk/gtkwindowprivate.h
@@ -66,8 +66,9 @@ void            _gtk_window_get_wmclass            (GtkWindow  *window,
                                                     gchar     **wmclass_name,
                                                     gchar     **wmclass_class);
 
-void            _gtk_window_set_allocation         (GtkWindow     *window,
-                                                    GtkAllocation *allocation);
+void            _gtk_window_set_allocation         (GtkWindow           *window,
+                                                    const GtkAllocation *allocation,
+                                                    GtkAllocation       *allocation_out);
 
 typedef void (*GtkWindowKeysForeachFunc) (GtkWindow      *window,
                                           guint           keyval,