‘Interacting with pips’ with option `margin: 0` removes highlight

[sphh]

I used the (Interacting with pip)[https://refreshless.com/nouislider/examples/#section-pips] as an example and set the slider's option margin to 0 (or deleted it). Now I can move the two handles on top of each other: A single pip is highlighted.

The problem is, when I again move on handle: The highlighting from the pip under the stationary handle is removed.

I added a PR #1303 to deal also with this case.