Visual elements get rearranged in relation to a slider element when converted to javascript/online

Description of the problem:
In one segment of my task I add visual elements (e.g. a polygon stimulus - black box) on top of a slider to cover the slider and to customize the design of my rating scale. On python, this works seamlessly, the element stays on top of the scale as participant responds and the slider pointer stays underneath the black box. In javascript, however, as soon as I respond on the sliding scale (and the pointer pops up), the black box that was on top of the slider disappears.

It would be great if you could let me know of a way I can achieve a full occlusion of the scale that does not change after the response is made.

URL of experiment: TBDTask [PsychoPy]