1.3.0

New features:

The offhand item is now stored in a player extended property when not assigned to an inventory slot, meaning many more mods that change the player's inventory are compatible with fewer conflicts.
A similar change to how the offhand slot container is implemented now allows more mods that change the player's inventory container to be compatible with fewer conflicts.
Most items are mirrored correctly in first and third person in the offhand now.
More items like fishing rods and thaumcraft wands are now usable through an added mixin and ticking server-side hotswap. The ticking hotswap is toggleable through the config.

Fixes:

Fixed ghost blocks when breaking blocks with the offhand in creative mode.
Fixed attacking an entity with the offhand right after using an item or interacting with an entity with an item.
Fixed server-side crash when checking for offhand use and shift placing offhand blocks on blocks with uses.
Fixed null pointer exception in offhand creative inventory.
NetHandlerPlayClient.handleSetSlot() mixin now sets item stacks to null in slots if their stack size is zero, fixing some container issues.
Fixed nullpointerexception client-side console spam on sent packets.
Server-sided configs are now synced with the client.

Backhand