Does new Callbacks overwrite Callback queue when full or they get rejected?

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When the Global Callback queue is full, if there is a new Callback coming in, would it overwrite the the old Callbacks or would the new Callback simply get dropped?

If it get dropped, what would be the best way to always get the latest message? I don't care if some old messages are drop. My application is that I want to get the current position of the robot. Even though the positioning system is updating the position at fast rate, the robot only want the read the latest position information periodically at a lower rate.