moveTo method
Moves the cursor (collapsed) to (fragmentId, localOffset). Resets preferredX: a click or horizontal movement cancels the column.
Implementation
void moveTo(String fragmentId, int localOffset, {bool forward = true}) {
preferredX = -1.0; // ← explicit reset
anchorId = fragmentId;
anchorOffset = localOffset;
focusId = fragmentId;
focusOffset = localOffset;
}