OpenGL paint engine crahses whenever QPainterPath's first element is not MoveElement. This can be achieved by using connectPath on PainterPath which contains only moveTo element.

/////////////////////////////////////////
Following example will clear the problem:

QPainterPath path1;
QPainterPath path2;
QPainterPath path3;
QPainterPath path4;

path1.moveTo(1,1);

path2.moveTo(4,4);
path2.lineTo(5,6);
path2.lineTo(6,7);

path3.connectPath(path2);

path4.lineTo(5,5);

path1.connectPath(path2);

qDebug() << path1;
qDebug() << path2;
qDebug() << path3;
qDebug() << path4;

////////////////////////////////////
The result is:

QPainterPath: Element count=3
-> LineTo(x=4, y=4)
-> LineTo(x=5, y=6)
-> LineTo(x=6, y=7)

QPainterPath: Element count=3
-> MoveTo(x=4, y=4)
-> LineTo(x=5, y=6)
-> LineTo(x=6, y=7)

QPainterPath: Element count=3
-> LineTo(x=4, y=4)
-> LineTo(x=5, y=6)
-> LineTo(x=6, y=7)

QPainterPath: Element count=2
-> MoveTo(x=0, y=0)
-> LineTo(x=5, y=5)

//////////////////////////////////////
As you may notice path1 and path3 are incorrect, since they don't have moveToElement as the first element in the path.

Using QPainter::drawPath(const QPainterPath & path) from paintEvent implemented in custom QGraphicsItem, while the view's viewport is a QGLWidget, will crash the application.

Q_ASSERT(m_elements.first().type == QPainterPath::MoveToElement); in void QStroker::processCurrentSubpath()