Just three suggestions that might improve the slider class.
Add a function with which you can retrieve the the handle being dragged.
Right now when ‘sliderValueChanged()’ is called there is no way to retrieve which of the possible three handles is being dragged.
The interval value which you can set with ‘setRange’ enables you to set the increment which is used when pressing the +/- buttons. However it allways rounds the value to a whole number. Which is not allways what you want.
So an option whether the interval actually constrains the value would be nice, something along these lines. setIntervalConstrainsValue(bool constrain)
Right now there is no way to make an inc/dec slider which accepts any number (within the range of course), without setting the interval to 0, which will deactivate the +/- buttons.
IncDec style drag
When dragging the +/- buttons to change the values the drag starts after the distance of the mouse changed at least 10 pixels. Which is good , but this 10 pixels is not taking in account when calculating the new value. Because after 10 pixels are dragged, the value first makes a big jump. So the 10 pixels have to be subtracted from the dragdistance again.
I understand this is not top priority but maybe this post helped you put up up to your todo list
And there are just minor changes.