This is a problem with Qt 4 on Mac/Cocoa, not a problem with WIndows or Linux versions.
If you embed a QGLWidget in a scrolled QWidget, and if the QGLWidget is not exposed when the QWidget is first shown, then you get an "invalid drawable" message and the QGLWIdget won't render. The peculiar behavior of Cocoa openGL contexts has been reported on Apple websites, e.g. http://lists.apple.com/archives/mac-opengl/2002/Mar/msg00189.html . However, Qt does not offer API to enable users to work around this. There is a nasty workaround that I found: You can hide() the QGLWidget and then call show() on it when it is first exposed. But Qt needs to offer a better fix.