To me, the point of the Adjust Drop Offset Distance hotkey is changing the target belt lane, which only has two options: far lane (inserter default) and close lane. Side Inserters handles this this by toggling between the farthest and closest dropoff points.

While cycling between all three options does work, the middle value is both mildly inconvenient and rather ambiguous; it's not obvious which lane it's going to use.

This feature is planned, I still haven't started working on it but I want it too so it will come for sure (maybe in the next update but I'm not too sure).

Done