Implement stack size and max damage restrictions

api/src/main/java/org/geysermc/geyser/api/item/custom/CustomItemData.java

@@ -120,6 +120,8 @@ public interface CustomItemData {
      *
      * Returns 0 if not set. When not set (or 0), it defaults to the stack count of the Java item when based of a vanilla item, or 64 when registering a non-vanilla item.
      *
+     * Note that, to copy Java behaviour, setting the stack size of an item to a value above 1 will set the max damage to 0. If a max damage value above 0 was explicitly set, an exception will be thrown.
+     *
      * @return the stack size of the item
      */
     @NonNegative
@@ -130,6 +132,8 @@ public interface CustomItemData {
      *
      * Returns -1 if not set. When not set (or below 0), it defaults to the maximum damage of the Java item when based of a vanilla item, or uses 0 when registering a non-vanilla item.
      *
+     * Note that, to copy Java behaviour, setting the max damage value of an item to a value above 0 will set the stack size to 1. If a stack size above 1 was explicitly set, an exception will be thrown.
+     *
      * @return the max damage of the item
      */
     int maxDamage();

core/src/main/java/org/geysermc/geyser/item/GeyserCustomItemData.java

@@ -275,12 +275,28 @@ public class GeyserCustomItemData implements CustomItemData {
 
         @Override
         public Builder stackSize(int stackSize) {
+            if (stackSize > 1) {
+                if (this.maxDamage > 0) {
+                    throw new IllegalStateException("Stack size cannot be above 1 when max damage is above 0");
+                }
+                // Explicitly set max damage to 0 instead of falling back to the Java vanilla item value
+                this.maxDamage = 0;
+            }
+
             this.stackSize = stackSize;
             return this;
         }
 
         @Override
         public Builder maxDamage(int maxDamage) {
+            if (maxDamage > 0) {
+                if (this.stackSize > 1) {
+                    throw new IllegalStateException("Max damage cannot be above 0 when stack size is above 1");
+                }
+                // Explicitly set stack size to 1 instead of falling back to the Java vanilla item value
+                this.stackSize = 1;
+            }
+
             this.maxDamage = maxDamage;
             return this;
         }

core/src/main/java/org/geysermc/geyser/registry/mappings/versions/MappingsReader_v1.java

@@ -222,7 +222,7 @@ public class MappingsReader_v1 extends MappingsReader {
         }
 
         if (node.has("attack_damage")) {
-            customItemData.maxDamage(node.get("attack_damage").asInt());
+            customItemData.attackDamage(node.get("attack_damage").asInt());
         }
 
         if (node.has("armor_type")) {