QCocoaWindow can return 0 devicePixelRatio

    // Crate large textures on high-dpi displays.
    if (sourceItem())
        textureSize *= d->window->devicePixelRatio();

    const QSize minTextureSize = d->sceneGraphContext()->minimumFBOSize();
    // Keep power-of-two by doubling the size.
    while (textureSize.width() < minTextureSize.width())
        textureSize.rwidth() *= 2; <- rwidth is 0 here.