It seems like there is a typo in the LookAndFeelDemo.cpp file of the JuceDemo. For the background of the horizontal slider in the custom look and feel to work, this code (lines 213-215):
if (slider.isHorizontal()) { const float iy = x + width * 0.5f - sliderRadius * 0.5f;
should actually read:
if (slider.isHorizontal()) { const float iy = y + height * 0.5f - sliderRadius * 0.5f;
Since the demo doesn't display horizontal sliders, this was easy to miss ;)
Should I have done a pull request for this?