Book and quill doesn't save changes when edited in offhand

The bug

When a book is signed in the off-hand, when switched back to the main hand, it reverts back to a Book and Quill.

How to reproduce

1. Get a Book and Quill
2. Switch Book and Quill to off-hand
3. Write in the book and sign it
4. Switch the book back to the main hand using the F hotkey

Code analysis

Based on 1.11 decompiled using MCP 9.35 rc1

The problem is that the method net.minecraft.network.NetHandlerPlayServer.processCustomPayload(CPacketCustomPayload) only tests for the mainhand when it receives a MC|BEdit or MC|BSign CPacketCustomPayload packet.