Shift-clicking in player inventory cannot move items into off-hand if item is no off-hand item

The bug

Shift-clicking an item which is not an offhand item (like a shield) will never move it to the offhand, even if no other slots are free or the offhand has already the same item.

How to reproduce

Option 1:

1. Place a chestplate in the chest armor slot

2. Fill your complete inventory except for the offhand slot with items

3. Try to shift-click the chestplate into the offhand slot
   → ❌ It does not work

Option 2:

1. Set your gamemode to survival

2. Put some torches in your offhand slot

3. Craft more torches and collect them using shift-click
   → ❌ Torches won't stack on the offhand slot but in the inventory

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

The method net.minecraft.inventory.ContainerPlayer.transferStackInSlot(EntityPlayer, int) only tries to merge items with items in slots up to (excluding) index 45 which is offhand. Therefore the items cannot be moved into the offhand.