Walk the overlay's text nodes in document order and find the text node
offset that corresponds to plain-text character position offset.
Offsets beyond the total text length clamp to the end of the last text
node. Returns { node: parent, offset: 0 } when there are no text nodes.
Walk the overlay's text nodes in document order and find the text node
offset. Offsets beyond the total text length clamp to the end of the last text node. Returns{ node: parent, offset: 0 }when there are no text nodes.