compute_drag_target_in_virtualized_list.dart

  void determineDragPosition(BuildContext itemContext,
      DragUpdateDetails details, Item draggedItem, double itemHeight) {
    _dragOperation.offset.value = details.globalPosition;

    // Find the fixed extent list renderer.
    RenderSliverFixedExtentList extentList;
    for (var p = itemContext.findRenderObject();
        p != null;
        p = p is RenderObject ? p.parent as RenderObject : null) {
      if (p is RenderSliverFixedExtentList) {
        extentList = p;
        break;
      }
    }
    if (extentList == null) {
      // We failed.
      return;
    }

    // We've got an extent list. Get the first child and find its top.
    var first = extentList.firstChild;
    // Compute the scroll offset of the first visible item (N.B. this is not
    // the first item in the list, it's the first one that's probably
    // visible).
    var offsetOfFirst = extentList.childScrollOffset(first);
    // We know this item's in the tree, so we can call globalToLocal on it.
    var local = first.globalToLocal(details.globalPosition);
    // Now we know where our cursor is relative to the first visible item.
    var y = offsetOfFirst + local.dy;
    // Compute index of item the cursor is over.
    var index = (y / itemHeight).floor();
    // Compute offset over that row (how close are we to the top of the row
    // we're hovering).
    var localOffset = y % itemHeight;

    // Use index to compute drop target.
    var dropTarget = index >= 0 && index < _flat.length ? _flat[index] : null;

    // Drop logic goes here
  }