fix: Intermittent Forgified Fabric API creative inventory crash on Forge 1.20.1

[arnabnandy7]

Summary

Fixes an intermittent creative inventory crash when Clutter No More is used with Forgified Fabric API and Sinytra Connector on Minecraft 1.20.1 Forge.

Fixes #27

Root cause

Clutter No More extends ItemStack equality so generated shape items and their parent items compare as equivalent.

However, ItemStackLinkedSet.TYPE_AND_TAG continued calculating hashes from the original item. This violated the required equality/hash contract:

• Shape and parent stacks compared equal.
• The same stacks produced different hashes.

Forge uses this strategy in MutableHashedLinkedMap when building creative inventory tabs. Depending on the map layout, Forgified Fabric API's iterator-removal pass could remove an unexpected entry and throw:

java.util.ConcurrentModificationException
at net.minecraftforge.common.util.MutableHashedLinkedMap$1.remove
at net.fabricmc.fabric.impl.itemgroup.ItemGroupEventsImpl.onCreativeModeTabBuildContents

The inconsistent map layout explains why the crash occurred intermittently.

Solution

• Normalize shape stacks to their parent before hashing.
• Normalize related stacks before comparing their tags.
• Preserve inequality when related stacks contain different NBT data.
• Restrict the compatibility implementation to Forge 1.20.1.
• Add a development-only before/after regression check.

Regression verification

The regression check recreates the previous broken hash strategy using actual registered item stacks.

Before

It verifies that shape and parent stacks compare equal while producing different hashes, then reproduces the reported:

ConcurrentModificationException

from MutableHashedLinkedMap.Iterator.remove().

After

It verifies that:

• Equal shape and parent stacks produce equal normalized hashes.
• Different NBT data remains unequal.
• Forgified Fabric API's iterator-removal pattern completes successfully.
• The map is empty after all entries are removed.

Successful runtime output:

[HashContractRegression] BEFORE reproduced: equal shape/parent stacks had different vanilla hashes; AFTER verified: normalized hashes match and FFAPI iterator removal completed

Validation

The development regression check was executed during a Forge 1.20.1 server launch.

The following builds pass:

./gradlew :1.20.1-forge:build
./gradlew :1.21.1-fabric:build
./gradlew :1.21.1-neoforge:build

Runtime verification:

./gradlew :1.20.1-forge:runServer