Fix ItemGroup API on servers

src/main/java/net/fabricmc/fabric/api/client/itemgroup/FabricItemGroupBuilder.java

@@ -92,12 +92,11 @@ public final class FabricItemGroupBuilder {
 	public ItemGroup build() {
 		((ItemGroupExtensions) ItemGroup.BUILDING_BLOCKS).fabric_expandArray();
 		return new ItemGroup(ItemGroup.GROUPS.length - 1, String.format("%s.%s", identifier.getNamespace(), identifier.getPath())) {
-			@Override
+
 			public ItemStack getIconItem() {
 				return stackSupplier.get();
 			}
 
-			@Override
 			public void getStacksForDisplay(DefaultedList<ItemStack> stacks) {
 				if(stacksForDisplay != null){
 					stacksForDisplay.accept(stacks);

src/main/java/net/fabricmc/fabric/mixin/client/itemgroup/MixinItemGroup.java

@@ -17,23 +17,16 @@
 package net.fabricmc.fabric.mixin.client.itemgroup;
 
 import net.fabricmc.fabric.client.itemgroup.FabricCreativeGuiComponents;
-import net.fabricmc.fabric.client.itemgroup.ItemGroupExtensions;
 import net.minecraft.item.ItemGroup;
 import org.spongepowered.asm.mixin.Final;
 import org.spongepowered.asm.mixin.Mixin;
-import org.spongepowered.asm.mixin.Mutable;
 import org.spongepowered.asm.mixin.Shadow;
 import org.spongepowered.asm.mixin.injection.At;
 import org.spongepowered.asm.mixin.injection.Inject;
 import org.spongepowered.asm.mixin.injection.callback.CallbackInfoReturnable;
 
 @Mixin(ItemGroup.class)
-public abstract class MixinItemGroup implements ItemGroupExtensions {
+public abstract class MixinItemGroup {
-
-	@Shadow
-	@Final
-	@Mutable
-	public static ItemGroup[] GROUPS;
 
 	@Shadow
 	public abstract int getId();
@@ -45,14 +38,6 @@ public abstract class MixinItemGroup implements ItemGroupExtensions {
 	@Final
 	private int id;
 
-	@Override
-	public void fabric_expandArray() {
-		ItemGroup[] tempGroups = GROUPS;
-		GROUPS = new ItemGroup[GROUPS.length + 1];
-		for (ItemGroup group : tempGroups) {
-			GROUPS[group.getId()] = group;
-		}
-	}
 
 	@Inject(method = "isTopRow", cancellable = true, at = @At("HEAD"))
 	private void isTopRow(CallbackInfoReturnable<Boolean> info) {

src/main/java/net/fabricmc/fabric/mixin/itemgroup/MixinItemGroup.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2016, 2017, 2018 FabricMC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package net.fabricmc.fabric.mixin.itemgroup;
+
+import net.fabricmc.fabric.client.itemgroup.ItemGroupExtensions;
+import net.minecraft.item.ItemGroup;
+import org.spongepowered.asm.mixin.Final;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Mutable;
+import org.spongepowered.asm.mixin.Shadow;
+
+@Mixin(ItemGroup.class)
+public abstract class MixinItemGroup implements ItemGroupExtensions {
+
+	@Shadow
+	@Final
+	@Mutable
+	public static ItemGroup[] GROUPS;
+
+	@Override
+	public void fabric_expandArray() {
+		ItemGroup[] tempGroups = GROUPS;
+		GROUPS = new ItemGroup[GROUPS.length + 1];
+		for (int i = 0; i < tempGroups.length; i++) {
+			GROUPS[i] = tempGroups[i];
+		}
+	}
+
+}

src/main/resources/net.fabricmc.fabric.mixins.common.json

@@ -17,6 +17,7 @@
     "events.server.MixinMinecraftServer",
     "events.tick.MixinMinecraftServer",
     "events.tick.MixinWorld",
+    "itemgroup.MixinItemGroup",
     "misc.MixinCrashReport",
     "networking.MixinServerPlayNetworkHandler",
     "networking.MixinSPacketCustomPayload",

src/test/java/net/fabricmc/fabric/itemgroup/ItemGroupMod.java

@@ -16,7 +16,7 @@
 
 package net.fabricmc.fabric.itemgroup;
 
-import net.fabricmc.api.ClientModInitializer;
+import net.fabricmc.api.ModInitializer;
 import net.fabricmc.fabric.api.client.itemgroup.FabricItemGroupBuilder;
 import net.minecraft.block.Blocks;
 import net.minecraft.item.Item;
@@ -27,9 +27,9 @@ import net.minecraft.util.Identifier;
 import net.minecraft.util.registry.Registry;
 
 
-public class ItemGroupMod implements ClientModInitializer {
+public class ItemGroupMod implements ModInitializer {
 	@Override
-	public void onInitializeClient() {
+	public void onInitialize() {
 		//This creates your standard Item Group
 		ItemGroup group = FabricItemGroupBuilder.build(new Identifier("fabric", "fabric_test_tab"), () -> new ItemStack(Items.IRON_CHESTPLATE));
 		Item testItem = new Item(new Item.Settings().itemGroup(group));