The Slider bar object provides a simple form control allowing the user to pick a value between a minimum and maximum by moving a slider along a bar.
Form controls are actual HTML elements floating above the game canvas in the HTML page. Therefore, nothing can be displayed on top of a form control, apart from other form controls.
Form controls position themselves according to the layer parallax, scale and so on. However they are not truly "in the game" (since they float above), so it is recommended to avoid using scrolling, parallax and layer scaling with form controls, otherwise the end result can feel unnatural.