Draw composited widget's content only after something is drawn.

views/widget/widget_gtk.cc

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "views/widget/widget_gtk.h"
 
 #include <gdk/gdk.h>
 #include <gdk/gdkx.h>
 #include <X11/extensions/shape.h>
 
@@ skipping 44 matching lines @@
     CompositePainter* painter = static_cast<CompositePainter*>(
         g_object_get_data(G_OBJECT(widget), kCompositePainterKey));
     if (!painter) {
       g_object_set_data(G_OBJECT(widget), kCompositePainterKey,
                         new CompositePainter(widget));
       g_signal_connect(widget, "destroy",
                        G_CALLBACK(&DestroyPainter), NULL);
     }
   }
 
-  // Enable the composition.
+  // Set the composition flag.
   static void SetComposited(GtkWidget* widget) {
-    DCHECK(GTK_WIDGET_REALIZED(widget));
-    gdk_window_set_composited(widget->window, true);
     g_object_set_data(G_OBJECT(widget), kCompositeEnabledKey,
                       const_cast<char*>(""));
   }
 
-  // Returns true if the |widget| is composited.
+  // Returns true if the |widget| is composited and ready to be drawn.
   static bool IsComposited(GtkWidget* widget) {
     return g_object_get_data(G_OBJECT(widget), kCompositeEnabledKey) != NULL;
   }
 
  private:
   virtual ~CompositePainter() {}
 
   // Composes a image from one child.
   static void CompositeChildWidget(GtkWidget* child, gpointer data) {
     GdkEventExpose* event = static_cast<GdkEventExpose*>(data);
@@ skipping 810 matching lines @@
     return;
   size_ = new_size;
   root_view_->SetBounds(0, 0, allocation->width, allocation->height);
   root_view_->SchedulePaint();
 }
 
 gboolean WidgetGtk::OnPaint(GtkWidget* widget, GdkEventExpose* event) {
   if (transparent_) {
     // Clear the background before drawing any view and native components.
     DrawTransparentBackground(widget, event);
+    if (type_ == TYPE_CHILD && !CompositePainter::IsComposited(widget_)) {
+      // Let the parent draw the content only after something is drawn on
+      // the widget.
+      CompositePainter::SetComposited(widget_);
+    }
   }
   root_view_->OnPaint(event);
   return false;  // False indicates other widgets should get the event as well.
 }
 
 void WidgetGtk::OnDragDataGet(GtkWidget* widget,
                               GdkDragContext* context,
                               GtkSelectionData* data,
                               guint info,
                               guint time) {
@@ skipping 387 matching lines @@
       // transparency has to be configured before widget is realized.
       DCHECK(parent) << "Transparent widget must have parent when initialized";
       ConfigureWidgetForTransparentBackground(parent);
     }
     gtk_container_add(GTK_CONTAINER(parent ? parent : null_parent_), widget_);
     gtk_widget_realize(widget_);
     if (transparent_) {
       // The widget has to be realized to set composited flag.
       // I tried "realize" signal to set this flag, but it did not work
       // when the top level is popup.
-      CompositePainter::SetComposited(widget_);
+      DCHECK(GTK_WIDGET_REALIZED(widget_));
+      gdk_window_set_composited(widget_->window, true);
     }
   } else {
     widget_ = gtk_window_new(
         (type_ == TYPE_WINDOW || type_ == TYPE_DECORATED_WINDOW) ?
         GTK_WINDOW_TOPLEVEL : GTK_WINDOW_POPUP);
     gtk_widget_set_name(widget_, "views-gtkwidget-window");
     if (transient_to_parent_)
       gtk_window_set_transient_for(GTK_WINDOW(widget_), GTK_WINDOW(parent));
     GTK_WIDGET_UNSET_FLAGS(widget_, GTK_DOUBLE_BUFFERED);
 
@@ skipping 198 matching lines @@
     GtkWindow* window = GTK_WINDOW(element->data);
     DCHECK(window);
     RootView *root_view = FindRootView(window);
     if (root_view)
       root_view->NotifyLocaleChanged();
   }
   g_list_free(window_list);
 }
 
 }  // namespace views