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/Viewer.cpp

@@ -617,6 +617,10 @@ void Viewer::eventTraversal()
     Contexts contexts;
     getContexts(contexts);
 
+    // set done if there are no windows
+    checkWindowStatus(contexts);
+    if (_done) return;
+
     osgGA::GUIEventAdapter* eventState = getEventQueue()->getCurrentEventState();
     osg::Matrix masterCameraVPW = getCamera()->getViewMatrix() * getCamera()->getProjectionMatrix();
     if (getCamera()->getViewport())