positionAt method
The stable Dot of the live element at visible index.
A Dot is a durable position: it keeps identifying the same element across concurrent edits (and survives the element's own deletion as a tombstone), so an editor pins cursors / selections / comments to it rather than to an integer index.
Implementation
Dot positionAt(int index) {
final vis = _visibleElems();
RangeError.checkValidIndex(index, vis, 'index', vis.length);
return vis[index].dot;
}