When shift-clicking an item in the inventory or in any container GUI, even if the item can stack with the item in the offhand slot, the game will not stack them.

For instance, if the player has torches in their offhand, and crafts more torches in the inventory or a crafting table, shift-clicking the crafted torches will place them in the hotbar or inventory rather than stacking them into the offhand slot.

This issue was previously reported nearly a year ago as MC-84345, but it was wrongly marked as a duplicate of MC-84224.