From Jan Peciva, "please find attached proposed fix for ON_DEMAND rendering. The biggest issue was

that the windows did not act on repaint request (WM_PAINT, EXPOSE,...)

Detailed explanation:
- I implemented requestRedraw using the push approach (not using
GraphicsWindow::_requestRedraw flag that I was considering) as there may be
multiple viewers reading the flag and fighting to reset it after the paint
request, while some viewers may not spot the request to redraw
- I made windows call GraphicsWindow::requestRedraw when they receive
appropriate message (WM_PAINT, EXPOSE, RESIZE,...)
- There were issues on Linux that windows did not want to close using x
button. Resolved by moving the test for DeleteWindow event from
swapBuffersImplementation() to GraphicsWindowX11::checkEvents(). The difficulty
was that DeleteWindow event is not coming using _eventDisplay, but through
_display.
- The last difficulty was that it is necessary to call
ViewerBase::checkWindowStatus() to set _done to true when all windows are
closed. This did not happened recently in ON_DEMAND run scheme. I put the call
to checkWindowStatus() to eventTraversal.
"

include/osgViewer/GraphicsWindow

@@ -30,6 +30,9 @@ extern "C"
 
 namespace osgViewer {
 
+class View;
+
+
 /** Base class for providing Windowing API agnostic access to creating and managing graphics window and events.
   * Note, the GraphicsWindow is subclassed from osg::GraphicsContext, and to provide an implemention you'll need to implement its
   * range of pure virtual functions, you'll find these all have naming convention methodNameImplemention(..).
@@ -180,8 +183,13 @@ class OSGVIEWER_EXPORT GraphicsWindow : public osg::GraphicsContext, public osgG
 
     public:
 
+        typedef std::list<osgViewer::View*> Views;
+        /** Returns the list of views (osgViewer::View) attached to this GraphicsWindow.
+         *  Internally, the method walks through all the cameras and collects all the views attached to the cameras.*/
+        void getViews(Views& views);
+
         // Override from GUIActionAdapter
-        virtual void requestRedraw() {}
+        virtual void requestRedraw();
 
         // Override from GUIActionAdapter
         virtual void requestContinuousUpdate(bool /*needed*/=true) {}