@Override
	public JComponent getHoverComponent(Program program, ProgramLocation programLocation,
			FieldLocation fieldLocation, Field field) {

		if (!enabled || programLocation == null) {
			return null;
		}

		Class<? extends ProgramLocation> clazz = programLocation.getClass();
		if (clazz != FunctionSignatureFieldLocation.class &&
			clazz != FunctionNameFieldLocation.class) {
			return null;
		}

		// is the label local to the function
		FunctionSignatureFieldLocation functionLocation =
			(FunctionSignatureFieldLocation) programLocation;

		Address entry = functionLocation.getFunctionAddress();
		FunctionManager functionManager = program.getFunctionManager();
		Function function = functionManager.getFunctionAt(entry);

		String toolTipText = ToolTipUtils.getToolTipText(function, true);
		JToolTip toolTip = new JToolTip();
		toolTip.setTipText(toolTipText);
		return toolTip;
	}
A Listing hover to show tool tips for function signatures
Ghidra/Features/Base/src/main/java/ghidra/app/plugin/core/codebrowser/hover/FunctionSignatureListingHover.java