private Function getFunctionAtAddress(Program program, Scalar scalar) {

		if (scalar == null) {
			return null;
		}

		long scalarLong = scalar.getValue();
		AddressFactory factory = program.getAddressFactory();
		AddressSpace space = factory.getDefaultAddressSpace();
		try {
			Address asAddress = factory.getAddress(space.getSpaceID(), scalarLong);
			return program.getListing().getFunctionAt(asAddress);
		}
		catch (AddressOutOfBoundsException ex) {
			return null; // scalar is not an address
		}
	}
A hover service to show tool tip text for hovering over a function name in the decompiler.
The tooltip shows the function signature per the listing.
Ghidra/Features/Decompiler/src/main/java/ghidra/app/decompiler/component/hover/FunctionSignatureDecompilerHover.java