In QGIS 3.4.8 I can label vertices with their vertex number by using the following expression for a Font marker symbol:

@geometry\_point\_num 

But, the expression only seems to count the vertices within the current view - so as I zoom in and out the numbers change. 

Is there a way to make this work for each whole polyline, irrespective of the view, so the numbers do not change? So I can, for example, zoom into a line and know I am looking at the fifth vertex along the line.