In case anyone arrives here from Google like I did, there’s an easy way to make a draggable number box without overriding LookAndFeelMethods or writing any custom classes, simply:
slider.setSliderStyle(Slider::SliderStyle::LinearBarVertical);
slider.setSliderSnapsToMousePosition(false);
slider.setColour(Slider::trackColourId, Colours::transparentBlack);
slider.setSize(60, 20);