Snapping to edge along one axis ??

I want to move the selected vertex up (green axis) to match the height of the diagonal edge.With snapping enabled and set to edge, I would expect it to snap to the edge. Instead, I can slide along the edge (left & right) and the vertex uses that height. Pretty much useless and I have no idea how to do what I want.
It drives me nuts!

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You have no vertex for the lower plane to snap to on the other plane. Edges won’t snap together unless they have matching vertices to snap to.

Select the upper edge that needs a vertex and subdivide it, then try again.