actually fix syncing to client (please)

src/main/java/gregtech/api/items/toolitem/ItemGTToolbelt.java

@@ -37,7 +37,6 @@
 import net.minecraft.util.math.RayTraceResult;
 import net.minecraft.world.IBlockAccess;
 import net.minecraft.world.World;
-import net.minecraftforge.common.ForgeHooks;
 import net.minecraftforge.common.capabilities.Capability;
 import net.minecraftforge.common.capabilities.ICapabilityProvider;
 import net.minecraftforge.common.util.INBTSerializable;
@@ -76,7 +75,8 @@
 
 public class ItemGTToolbelt extends ItemGTTool implements IDyeableItem {
 
-    private static final ThreadLocal<Integer> slotThread = ThreadLocal.withInitial(() -> -999);
+    private static final ThreadLocal<Integer> lastSlot = ThreadLocal.withInitial(() -> -999);
+    private static final ThreadLocal<EntityPlayer> lastPlayer = ThreadLocal.withInitial(() -> null);
 
     public ItemGTToolbelt(String domain, String id, Supplier<ItemStack> markerItem, IToolBehavior... behaviors) {
         super(domain, id, -1,
@@ -391,8 +391,8 @@ public void setOnCraftIngredient(ItemStack stack, Ingredient ingredient) {
         int match = getHandler(stack).checkIngredientAgainstTools(ingredient);
         if (match != -1) {
             setSelectedTool(match, stack);
-            PacketToolbeltSelectionChange.toClient(match, slotThread.get(),
-                    (EntityPlayerMP) ForgeHooks.getCraftingPlayer());
+            PacketToolbeltSelectionChange.toClient(match,
+                    lastSlot.get(), (EntityPlayerMP) lastPlayer.get());
         }
     }
 
@@ -525,8 +525,9 @@ public static boolean checkIngredientAgainstToolbelt(@NotNull ItemStack input, @
         return false;
     }
 
-    public static void setCraftingSlot(int slot) {
-        slotThread.set(slot);
+    public static void setCraftingSlot(int slot, EntityPlayer player) {
+        lastSlot.set(slot);
+        lastPlayer.set(player);
     }
 
     public static boolean checkToolAgainstToolbelt(@NotNull ItemStack toolbelt, @NotNull ItemStack tool) {

src/main/java/gregtech/core/network/packets/PacketToolbeltSelectionChange.java

@@ -39,6 +39,7 @@ public void decode(PacketBuffer buf) {
     }
 
     public static void toClient(int slot, int matrixSlot, EntityPlayerMP player) {
+        if (player == null) return;
         GregTechAPI.networkHandler.sendTo(new Client(slot, matrixSlot), player);
     }
 

src/main/java/gregtech/mixins/minecraft/ContainerMixin.java

@@ -0,0 +1,29 @@
+package gregtech.mixins.minecraft;
+
+import gregtech.api.items.toolitem.ItemGTToolbelt;
+
+import net.minecraft.entity.player.EntityPlayer;
+import net.minecraft.inventory.ClickType;
+import net.minecraft.inventory.Container;
+import net.minecraft.item.ItemStack;
+
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfoReturnable;
+
+@Mixin(Container.class)
+public abstract class ContainerMixin {
+
+    @Inject(method = "slotClick",
+            at = @At(value = "INVOKE",
+                     target = "Lnet/minecraft/item/ItemStack;splitStack(I)Lnet/minecraft/item/ItemStack;",
+                     ordinal = 1))
+    private void setPlayer(int slotId, int dragType, ClickType clickTypeIn, EntityPlayer player,
+                           CallbackInfoReturnable<ItemStack> cir) {
+        var playerStack = player.inventory.getItemStack();
+        if (playerStack.getItem() instanceof ItemGTToolbelt) {
+            ItemGTToolbelt.setCraftingSlot(slotId, player);
+        }
+    }
+}

src/main/resources/mixins.gregtech.minecraft.json

@@ -9,7 +9,7 @@
   },
   "mixins": [
     "BlockConcretePowderMixin",
-    "NetworkHandlerMixin",
+    "ContainerMixin",
     "BlockRenderLayerMixin",
     "DamageSourceMixin",
     "EnchantmentCanApplyMixin",