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.
"

src/osgViewer/GraphicsWindow.cpp

@@ -0,0 +1,60 @@
+/* -*-c++-*- OpenSceneGraph - Copyright (C) 1998-2011 Robert Osfield
+ *
+ * This library is open source and may be redistributed and/or modified under
+ * the terms of the OpenSceneGraph Public License (OSGPL) version 0.0 or
+ * (at your option) any later version.  The full license is in LICENSE file
+ * included with this distribution, and on the openscenegraph.org website.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * OpenSceneGraph Public License for more details.
+*/
+
+#include <osgViewer/GraphicsWindow>
+#include <osgViewer/View>
+#include <osgViewer/ViewerBase>
+
+using namespace osgViewer;
+
+
+void GraphicsWindow::getViews(Views& views)
+{
+    views.clear();
+    osgViewer::View *prev = NULL;
+
+    for(Cameras::iterator it = _cameras.begin();
+        it != _cameras.end();
+        it++)
+    {
+        osgViewer::View *v = dynamic_cast<osgViewer::View*>((*it)->getView());
+        if (v)
+            // perform a simple test to reduce the number of duplicates
+            if (v != prev)
+                // append view
+                views.push_back(v);
+    }
+
+    // remove duplicates
+    views.sort();
+    views.unique();
+}
+
+void GraphicsWindow::requestRedraw()
+{
+    Views views;
+    getViews(views);
+
+    if (views.empty())
+    {
+        OSG_INFO << "GraphicsWindow::requestRedraw(): No views assigned yet." << std::endl;
+        return;
+    }
+
+    for(Views::iterator it = views.begin();
+        it != views.end();
+        it++)
+    {
+        (*it)->requestRedraw();
+    }
+}