Custom QWidget Not Showing in QMainWindow (Qt4.8) - c++

Question

Summary: I wish to use X11 to paint a custom QWidget. It works unless that widget is in a layout or QMainWindow
I have a custom widget derived from QWidget that I'd like to be the main widget in a QMainWindow. When I run something like this:
int main(int argc, char** argv) {
QApplication app(argc, argv);
ModelWidget mw;
mw.show();
return app.exec();
}
everything works fine, including resizing, obscuring and revealing the window contents, etc.
However, if I try to use that widget as the central widget in a QMainWindow, nothing is painted in the central widget area of the QMainWindow.
Here's the constructor of the QMainWindow:
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent)
{
modelwidg = new ModelWidget;
setCentralWidget(modelwidg);
createActions();
createMenus();
}
I get the feeling that there's something related to the size or resize policy of my custom widget that I need to implement, but I can't find any documentation regarding what functions must be provided by a widget for it to be usable as a central widget in a QMainWindow. What am I missing?
Edit: Here's the custom widget
ModelWidget::ModelWidget(QWidget *parent) :
QWidget(parent)
{
setAttribute(Qt::WA_PaintOnScreen);
setAttribute(Qt::WA_NoSystemBackground);
setAttribute(Qt::WA_OpaquePaintEvent);
setAttribute(Qt::WA_NativeWindow);
setAutoFillBackground(true);
const QX11Info &info = x11Info();
// Elided X11 and glX specific stuff
create(wnd, true, true);
}
void ModelWidget::paintEvent(QPaintEvent *)
{
// scene.render is just some OpenGL stuff
scene->render();
glXSwapBuffers(dpy, glxwnd);
}
void ModelWidget::resizeEvent(QResizeEvent * e)
{
glViewport(0, 0, e->size().width(), e->size().height());
scene->set_aspect(float(e->size().width()) / float(e->size().height()));
update();
}
QSize ModelWidget::sizeHint() const
{
return QSize(640, 480);
}
Further...
According to the docs,
To render outside of Qt's paint system, e.g., if you require native
painting primitives, you need to reimplement QWidget::paintEngine() to
return 0 and set [Qt::WA_PaintOnScreen].
I've done that, but the window still remains unpainted. I suspect that one of my X11 objects, Window, Display*, is being altered by adding this widget to a layout or MainWindow.