V2.6 Stable Release:

Took some time to work out the bugs so this should be extremely stable until version 2.7 is finished ( a large update ).
Please make sure your CraftBukkit or Spigot 1.9 version is up to date with /version! If it is not up to date ItemJoin may not work!
Offhand support is fairly new so if you find any bugs please report them!

Added:

• Added Offhand item support. Definable as slot 104:. Reference: ( 1 - 36 is main inventory. ) ( 100 - 103 are armor slots. ) ( 104 is the offhand slot. )
  

Fixed:

• Possibly fixed all skull item lag.
  
• Players bypassing inventory-modify prevention using the "F" key. ( Moving the item to the offhand slot ).
  
• Skull item RIGHT_CLICK_BLOCK action not working.
  
• Fixed the CraftItemStack dependency. ItemJoin should now work with ALL future versions of CraftBukkit/Spigot and should not break.
  
• Fixed EntityDamagedByEntityEvent Errors.
  
• Fixed EntityShootBowEvent Errors.
  

Have any ideas or requests that you would like to see in ItemJoin's future? Please submit a feature request!

Skull Items still cause a bit of lag.