How to replicate

• Create a custom inventory with the following code

Inventory inventory = Bukkit.getServer().createInventory(player, 54, "DemoGui");
inventory.setItem(0, new ItemStack(Material.APPLE));
player.openInventory(inventory);

• Cancel every type of InventoryClickEvent 

@EventHandler
public void inventoryClickEvent(InventoryClickEvent event) {
    event.setCancelled(true);
    System.out.println(event.getAction().toString() + " cancelled.");
}

• Observe that clicking on the apple with your mouse is correctly cancelled and the item does not move.

• Move your mouse of the item and press "F"
• Observe that the inventory item is copied to the offhand slot

Expected behaviour:

The item does not get copied to the offhand slot if the InventoryClickEvent is cancelled.