My slider is in a fieldSet and the trackWidth value of the slider is automaticaly setted to a bigger value than the slider's witdh. Even if I set another value to the trackWidth the false value is setted when the slider is rendered.
something like this and I found that the trackWidth of the slider was automaticaly bigger than the slider's width even if I set "trackWidth: some pixels". but when the window is repainted ( by resizing window) the value become correct.
But it's ok I setted the slider's width in CSS and it works now.