Distinguish drop down selection vs tapNodeData in dash cytoscape

I am developing a dashboard to visualize and explore a graph. I have a drop down selection to select a starting node and have callbacks to progressively expand the graph from this example dash-cytoscape/usage-elements.py at master · plotly/dash-cytoscape · GitHub

Currently my callback looks like

    Output("cytoscape-nodes", "elements"),
        Input("node-selection-dropdown", "value"),
        Input('cytoscape-two-nodes', 'tapNodeData')
    [State('cytoscape-nodes', 'elements'),
     State('radio-expand', 'value')]
def filter_nodes(node, nodeData, elements, explore_mode):

Right now there is no way to figure out if a new node has been selected from the dropdown or an existing node has been selected. I get values in both node and nodeData parameters.