The idea is: in a List widget with fields (e.g. not "simple string list"), one could add a hidden field that holds the index of the current item in that list, like:

list:
- fruit: apple
  index: 0 # <---
- fruit: banana
  index: 1 # <---

It could be a hidden widget with default: '{{index}}', or a compute widget with value: '{{index}}', or even something else...