interest in a "getFractionalVisibleRange" method:

[tnrich]

In my local fork of react list I wrote a helper method based on getVisibleRange. It is useful for me because I'm making a minimap of the whole list that needs to show exactly what is being shown in the list. This is contrived and fake data, but I think you can see the general idea:

The blue highlight layer is tracking the scroll percentage and to get the fraction of the scroll, I use getFractionalVisibleRange:

Here it is:

getFractionalVisibleRange() {
    const { from, size } = this.state;
    const { start, end } = this.getStartAndEnd(0);
    const cache = {};
    let first, last;

    for (let i = from; i < from + size; ++i) {
      const itemStart = this.getSpaceBefore(i, cache);
      const itemEnd = itemStart + this.getSizeOf(i);
      if (first == null && itemEnd > start)
        first = i + 1 - (itemEnd - start) / (itemEnd - itemStart);
      if (first != null && itemStart < end)
        last = i - (itemEnd - end) / (itemEnd - itemStart);
    }
    return [first, last];
  }

Anyways, I thought it might be useful to someone else or could maybe even be added to the library.

@caseywebdev