SlotItemHandler#getMaxStackSize call IItemHandlerModifiable#setStackInSlot

[reasure3]

Minecraft Version: 1.21.1

NeoForge Version: 21.1.97

Steps to Reproduce:

1. make SlotItemHandler in your menu class
2. Place 32 dirt in the slot
3. Add 1 dirt to the slot
4. IItemHandler#setStackInSlot is called 3 times (First: Air, Second: Dirt * 32, Third: Dirt * 33)
5. This creates unintended side effects.

Description of issue:
Below is a portion of the SlotItemHandler#getMaxStackSize code.

@Override
public int getMaxStackSize(ItemStack stack) {
    ItemStack maxAdd = stack.copy();
    int maxInput = stack.getMaxStackSize();
    maxAdd.setCount(maxInput);

    IItemHandler handler = this.getItemHandler();
    ItemStack currentStack = handler.getStackInSlot(index);
    if (handler instanceof IItemHandlerModifiable) {
        IItemHandlerModifiable handlerModifiable = (IItemHandlerModifiable) handler;

        handlerModifiable.setStackInSlot(index, ItemStack.EMPTY);

        ItemStack remainder = handlerModifiable.insertItem(index, maxAdd, true);

        handlerModifiable.setStackInSlot(index, currentStack);

        return maxInput - remainder.getCount();
    }

As you can see, there is a part

handlerModifiable.setStackInSlot(index, ItemStack.EMPTY);
handlerModifiable.setStackInSlot(index, currentStack);

This creates unintended side effects.
For example, IItemHandler#onContentChanged is called a total of 3 times.
In my case, I want to update the recipe when a different item is inserted, just like the StoneCutter block works.
However, it didn't work properly because ItemStack.Empty is always inserted once when an item is changed in the slot.

[KnightMiner]

This has been the case for awhile, I suspect Forge was mimicking some vanilla logic or otherwise forgot they had their own method for this. I have a fix that I implemented in my own mods here: https://github.com/SlimeKnights/Mantle/blob/1.20/src/main/java/slimeknights/mantle/inventory/SmartItemHandlerSlot.java

Easy enough to add to a mod or to just fix in Neo, I don't think there are any advantages to doing it the weird vanilla way.