anchorFirst = () => {
    this.startRow = anchorRow;
    this.startNode = selection.anchorNode;
    this.startOffset = selection.anchorOffset;
    this.endRow = focusRow;
    this.endNode = focusNode;
    this.endOffset = focusOffset;
  }
Synchronize this object with the current DOM selection.

This is a one-way synchronization, the DOM selection is copied to this
object, not the other way around.