QQuickFramebufferObject is a QSGTextureProvider, which is a very nice/useful optimization, but leads to unexpected behavior when mirrorVertically is true, because ShaderEffect/QtGraphicalEffects do not know about the need to mirror.

An easy "fix" would be to turn off this optimization in that case. But I think this is a generic deficiency with texture providers (e.g. see also https://bugreports.qt.io/browse/QTBUG-77396) that could perhaps be improved by additional API in Qt 6. For example, what about adding a bool (or expandable set of flags) to QSGTextureProvider that allowed for more context about the texture being provided to the ShaderEffect?

Another issue is that smooth is not respected when QQuickFramebufferObject is being used as a QSGTextureProvider