At the moment, if one calls QSlider::initStyleOption it doesn't fill activeSubControls, although it does that before paint inside paintEvent function.
Is there any specific reason not to do so?
I've changed this logic a bit as being shown in the attached patch file and it works fine.
Please let me know if this is okay to change this function like this.

P.S I came across this issue during custom paintEvent implementation there I was needed to know whenever slider handle is the active control.