@r-tea, subclass the StringObject and add an event handler catching IDCMP_RAWKEY.
See http://www.sasg.com/mui/autodocs/MUI_Window.html#MUIM_Window_AddEventHandler for the details.

That way, you'll be able to see all keyboard events and catch UP/DOWN keys. If you want to eat the event (ie no further processing), the EventHandler method should return MUI_EventHandlerRC_Eat.