With 'Fusion' style, a disabled QSlider is drawn like it is enabled.

You can test this with the gallery example (examples/widgets/gallery), where you can set the style to 'Fusion' and disable the widgets.
See the attached screenshot below.

The distinction simply seems to be missing in qfusionstyle.cpp.
I attach a quick patch (slider_fusion_patch.diff), which solves the problem by don't drawing the groove blue and with a gradient, if the widget isn't in an enabled state.