move_item_down.py

v = iface.layerTreeView()
lyr_tree_model = v.layerTreeModel()
tree = lyr_tree_model.rootGroup()
selection_model = v.selectionModel()
current_index = selection_model.currentIndex()
row = current_index.row()
    
def get_first_ancestor_which_is_not_last_child(n):
    '''Recursive method the first ancestor of the current node
    which is not the last child of it's parent'''
    while v.node2index(n).row() == len(n.parent().children())-1:
        n = get_first_ancestor_which_is_not_last_child(n.parent())
    return n

def move_down(node):
    pn = node.parent()
    if not isinstance(pn, QgsLayerTree) or v.node2index(node).row() < len(pn.children())-1:
        if v.node2index(node).row() < len(pn.children())-1:
            sibling_below = pn.children()[row+1]
            # If sibling below is a group, insert node into that group at index 0
            if isinstance(sibling_below, QgsLayerTreeGroup):
                sibling_below.insertChildNode(0, node.clone())
                selection_model.setCurrentIndex(v.node2index(sibling_below.children()[0]), QItemSelectionModel.Select)
            # If sibling below is not a group, insert node into parent at index below
            else:
                pn.insertChildNode(row+2, node.clone())
                selection_model.setCurrentIndex(v.node2index(pn.children()[row+2]), QItemSelectionModel.Select)
        else:# node is last child of its parent
            #We get the first ancestor of the current node which is not the last child of its parent,
            #get the sibling below of that ancestor and insert the node into the parent of that ancestor, above the sibling below
            target_ancestor = get_first_ancestor_which_is_not_last_child(node)
            pnp = target_ancestor.parent()
            node_below = pnp.children()[v.node2index(target_ancestor).row()+1]
            node_below.parent().insertChildNode(v.node2index(node_below).row(), node.clone())
            selection_model.setCurrentIndex(v.node2index(node_below.parent().children()[v.node2index(node_below).row()-1]), QItemSelectionModel.Select)
        pn.removeChildNode(node)
    
n = v.index2node(current_index)
if n:
    move_down(n)