A surface’s implied height above the page, communicated by its shadow: the higher the surface, the larger, softer, and farther the shadow falls.
Give the dropdown more elevation than the card it opens from. Right now they read as the same layer.
Elevation turns shadow into hierarchy along the z-axis: what floats above what, which layer owns the moment. Material Design’s real contribution was rationing it: fixed levels instead of a free shadow slider, one imagined light source for the whole interface. Two things break the illusion: decorative shadows under things that sit above nothing, and dark mode, where shadow has nothing darker to fall on and height must be restated as a lighter surface.