For HiPS cubes, Firefly is displaying the 3rd coordinate for each plane as user click on the left/right arrow to change plane. When there are hundreds of planes in a cube, it is cumbersome/slow to click through each plane. This ticket suggest a better design in UI to allow users to quickly get to the plane they want to see.
It would be very elegant indeed to visualize the value as a slider, with change-image-on-mouse-release behavior, of course, because the image load is probably way too expensive to do on mouse-move or even on mouse-pause. (IRSA-1629)