delete method
Tombstone the live element at visible index i. Returns the start dot of
the affected block, or null when i is out of range.
Implementation
Dot? delete(int i) {
final vis = _visibleElems();
if (i < 0 || i >= vis.length) return null;
final e = vis[i];
e.block.deleted.add(e.offset);
vis.removeAt(i);
return e.block.start;
}