Added _prefill function argument. According to your needs, you can change, if the last slice gets filled, if the given value is >= sliceStart or sliceEnd.
On a default progress bar with 5 slices (20% per slice), a given value of 50% and _prefill=false, 2 of 5 slices gets filled. With _prefill=true you get 3 of 5 slices filled.