How does the offhand keybind from this mod differ from the vanilla swap-hands keybind? 

I just came back from a Survival mining run carrying 4 leather backpacks filled (along with my main items bar) and realized that some of the items I was Shift+Clicking from killed mobs to my inventory just went... somewhere else I guess... They moved away from the mob's recovered inventory but I figured they were not matching any existing item in my inventory and just move to somewhere I couldn't recover them. Idk if there's a secret backpack where these items moved to but my inventory just doesnt show these recently acquired items. Before installing this mod today, the items would just not transfer to the player's inventory from a killed mob inventory if there was no more space available. And before you ask, no the items were not dropped on the floor either. They just go somewhere into oblivion. 

I've been wanting a mod that does this for ages, this is awesome

I'm not sure if this is intended behavior, but when I press B on an item (like a spyglass from the Spyglass mod) it places it into one of the 4 bag slots, when normally this is not possible.

I actually really think this is cool, and like being able to use those slots for "gear" type stuff, at least for roleplay sake. If this is unintended I think it might be worth keeping because it kinda feels neat to have them almost as pockets as well as for bags. Maybe have a different hotkey (shift-B) to do this intentionally.

I was thinking of also adding Held Item to offhand/Swap held item with offhand. Would anyone be interested in this as well?

It's both. Clientside for the keybind and request which is handed off to the server to move the items.

is it client side?