Implementing updatePaintNode causes other QML elements to also draw incorrectly

Question

I have discovered that geometry->setLineWidth(3); in the code below extends to other QML elements and can distort them, even if those other QML elements are "normal" QML elements (with no QQuickItem subclass beneath them). This seems odd to me that you could affect other elements and I wonder if it is a bug? The documentation says that this function should only affect the current element, but that is not my experience. Can anyone weigh in on why geometry->setLineWidth(3); has such unwieldy power?

QSGNode *updatePaintNode(QSGNode *oldNode, UpdatePaintNodeData *data){
    QSGGeometry *geometry = 
                 new QSGGeometry(QSGGeometry::defaultAttributes_Point2D(), 2);
    geometry->setDrawingMode(GL_LINES);
    geometry->setLineWidth(3);
    geometry->vertexDataAsPoint2D()[0].set(0, 0);
    geometry->vertexDataAsPoint2D()[1].set(width(), height());

    QSGFlatColorMaterial *material = new QSGFlatColorMaterial;
    material->setColor(QColor(255, 0, 0));

    QSGGeometryNode *node = new QSGGeometryNode;
    node->setGeometry(geometry);
    node->setFlag(QSGNode::OwnsGeometry);
    node->setMaterial(material);
    node->setFlag(QSGNode::OwnsMaterial);

    delete oldNode;
    return node;
}

If I omit the line geometry->setLineWidth(3); then the problem goes away.

Answer 1

I think it's a bug, sort of. The implementation in gsgbatchrenderer.cpp looks like this:

if (g->drawingMode() == GL_LINE_STRIP || g->drawingMode() == GL_LINE_LOOP || g->drawingMode() == GL_LINES)
        glLineWidth(g->lineWidth());

The gl functions are global on the current OpenGL context so they apply to all drawing commands, and there's nothing that sets glLineWidth back to its previous value. However, any other nodes that also are in GL_LINES mode will also set the line width when it's their turn to render. So I'm guessing the other QML elements must be rendering lines without going through this code and setting the glLineWidth themselves. If I'm right then that's the bug and those other elements need to explicitly set the width.

Can you tell us what type those other 'normal' QML elements are?