From Stephan Huber and Robert Osfield,

Stephan: "attached you'll find some modifications to the GraphicsWindow-class and
their platform-dependant implementations.

The problem:
setWindowRectangle and setWindowDecoration do not update the
traits-object, so, if you call setWindowRectangle on a
not-realized-window it will open with another size when realized later.
getWindowRectangle reports possible wrong sizes if setWindowRectangle
called before.

My solution:
split the implementation in two parts:
GraphicsWindow::setWindowRectangle will update its traits-object and
call afterwards the virtual method setWindowRectangleImplementation
(which is implemented by the derived platformspecific classess). For
setWindowDecoration I am useing a similar mechanism.

I hope you'll find the submission useful, the Win32 and X11 changes are
not tested but should work."

Changes to this made by Robert are call of resized in setWindowRectangle 
instead of setting of Traits, and use of a bool return type.

src/osgViewer/GraphicsWindowX11.cpp

@@ -292,7 +292,7 @@ bool GraphicsWindowX11::createVisualInfo()
     return _visualInfo != 0;
 }
 
-void GraphicsWindowX11::setWindowDecoration(bool flag)
+bool GraphicsWindowX11::setWindowDecorationImplementation(bool flag)
 {
     Display* display = getDisplayToUse();
 
@@ -361,15 +361,22 @@ void GraphicsWindowX11::setWindowDecoration(bool flag)
         // we don't add this sleep then any X11 calls right afterwards can produce
         // X11 errors.
         usleep(100000);
+        
+        return true;
 
     }
     else
+    {
         osg::notify(osg::NOTICE)<<"Error: GraphicsWindowX11::setBorder(" << flag << ") - couldn't change decorations." << std::endl;
+        return false;
+    }
+
+    
 }
 
-void GraphicsWindowX11::setWindowRectangle(int x, int y, int width, int height)
+bool GraphicsWindowX11::setWindowRectangleImplementation(int x, int y, int width, int height)
 {
-    if (!_realized) return;
+    if (!_realized) return false;
     
     Display* display = getDisplayToUse();
     
@@ -382,6 +389,8 @@ void GraphicsWindowX11::setWindowRectangle(int x, int y, int width, int height)
     // we don't add this sleep then any X11 calls right afterwards can produce
     // X11 errors.
     usleep(100000);
+    
+    return true;
 }
 
 void GraphicsWindowX11::setCursor(MouseCursor mouseCursor)
@@ -1291,4 +1300,7 @@ struct RegisterWindowingSystemInterfaceProxy
 RegisterWindowingSystemInterfaceProxy createWindowingSystemInterfaceProxy;
 
 // declare C entry point for static compilation.
-extern "C" void graphicswindow_X11(void) {}
+extern "C" void graphicswindow_X11(void)
+{
+    osg::GraphicsContext::setWindowingSystemInterface(new X11WindowingSystemInterface);
+}