perf: read inventory holders without copying the block

src/main/java/net/tfminecraft/activitytf/gui/ActivityGui.java

@@ -328,7 +328,7 @@ private ItemStack item(ItemStack stack, String name, List<String> lore) {
 
     @EventHandler
     public void onClick(InventoryClickEvent event) {
-        if (!(event.getView().getTopInventory().getHolder() instanceof Marker)) {
+        if (!(event.getView().getTopInventory().getHolder(false) instanceof Marker)) {
             return;
         }
         event.setCancelled(true);
@@ -475,7 +475,7 @@ private void claim(Player player, InventoryClickEvent event) {
 
     @EventHandler
     public void onDrag(InventoryDragEvent event) {
-        if (!(event.getView().getTopInventory().getHolder() instanceof Marker)) {
+        if (!(event.getView().getTopInventory().getHolder(false) instanceof Marker)) {
             return;
         }
         for (int rawSlot : event.getRawSlots()) {

src/main/java/net/tfminecraft/activitytf/gui/ReferralGui.java

@@ -602,7 +602,7 @@ public void confirmTyped(Player player, String name) {
 
     @EventHandler
     public void onClick(InventoryClickEvent event) {
-        if (!(event.getView().getTopInventory().getHolder() instanceof Marker marker)) {
+        if (!(event.getView().getTopInventory().getHolder(false) instanceof Marker marker)) {
             return;
         }
         event.setCancelled(true);
@@ -764,7 +764,7 @@ static String linkKey(Link link) {
 
     @EventHandler
     public void onDrag(InventoryDragEvent event) {
-        if (!(event.getView().getTopInventory().getHolder() instanceof Marker)) {
+        if (!(event.getView().getTopInventory().getHolder(false) instanceof Marker)) {
             return;
         }
         int size = event.getView().getTopInventory().getSize();

src/main/java/net/tfminecraft/activitytf/managers/ActivityManager.java

@@ -788,7 +788,7 @@ public boolean runClickCommands(Player player, int slot) {
 
     void runClickCommandsNow(Player player, List<String> commands, String id, Predicate<String> console) {
         InventoryView open = player.getOpenInventory();
-        if (open != null && open.getTopInventory().getHolder() instanceof ActivityGui.Marker) {
+        if (open != null && open.getTopInventory().getHolder(false) instanceof ActivityGui.Marker) {
             player.closeInventory();
         }
 

src/test/java/net/tfminecraft/activitytf/testsupport/HolderLookupPatch.java

@@ -0,0 +1,91 @@
+package net.tfminecraft.activitytf.testsupport;
+
+import static net.bytebuddy.matcher.ElementMatchers.is;
+import static net.bytebuddy.matcher.ElementMatchers.named;
+import static net.bytebuddy.matcher.ElementMatchers.takesArguments;
+
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import net.bytebuddy.agent.ByteBuddyAgent;
+import net.bytebuddy.agent.builder.AgentBuilder;
+import net.bytebuddy.asm.Advice;
+import net.bytebuddy.description.type.TypeDescription;
+import net.bytebuddy.dynamic.DynamicType;
+import net.bytebuddy.utility.JavaModule;
+import org.bukkit.inventory.Inventory;
+import org.bukkit.inventory.InventoryHolder;
+import org.junit.platform.launcher.LauncherSession;
+import org.junit.platform.launcher.LauncherSessionListener;
+import org.mockbukkit.mockbukkit.inventory.InventoryMock;
+
+/**
+ * MockBukkit throws on Paper's {@code getHolder(false)}. Its inventories hold no block snapshot,
+ * so the patch answers with the plain holder, which is what Paper returns for plugin GUIs.
+ */
+public final class HolderLookupPatch implements LauncherSessionListener {
+    private static boolean installed;
+
+    @Override
+    public void launcherSessionOpened(LauncherSession session) {
+        install();
+    }
+
+    /** Patches {@link InventoryMock#getHolder(boolean)} once per test JVM. */
+    public static synchronized void install() {
+        if (installed) {
+            return;
+        }
+        AtomicBoolean patched = new AtomicBoolean();
+        AtomicBoolean rejected = new AtomicBoolean();
+        new AgentBuilder.Default()
+                .disableClassFormatChanges()
+                .with(AgentBuilder.RedefinitionStrategy.RETRANSFORMATION)
+                .with(AgentBuilder.InitializationStrategy.NoOp.INSTANCE)
+                .with(AgentBuilder.TypeStrategy.Default.REDEFINE)
+                .with(AgentBuilder.Listener.StreamWriting.toSystemError().withErrorsOnly())
+                .with(new AgentBuilder.Listener.Adapter() {
+                    @Override
+                    public void onTransformation(TypeDescription type, ClassLoader loader, JavaModule module,
+                            boolean loaded, DynamicType dynamicType) {
+                        if (type.getName().equals(InventoryMock.class.getName())) {
+                            patched.set(true);
+                        }
+                    }
+
+                    // The JVM can still reject the new bytes after onTransformation.
+                    @Override
+                    public void onError(String typeName, ClassLoader loader, JavaModule module, boolean loaded,
+                            Throwable throwable) {
+                        if (typeName.equals(InventoryMock.class.getName())) {
+                            rejected.set(true);
+                        }
+                    }
+                })
+                .type(is(InventoryMock.class))
+                .transform((builder, type, loader, module, domain) -> builder.visit(Advice.to(PlainHolder.class)
+                        .on(named("getHolder").and(takesArguments(boolean.class)))))
+                .installOn(ByteBuddyAgent.install());
+        // A failed retransformation only reaches the listeners, so check it took before marking it done.
+        if (!patched.get() || rejected.get()) {
+            throw new IllegalStateException("Could not patch InventoryMock#getHolder(boolean)");
+        }
+        installed = true;
+    }
+
+    /** Skips the throwing body and returns {@code getHolder()} instead. */
+    static final class PlainHolder {
+        private PlainHolder() {
+        }
+
+        @Advice.OnMethodEnter(skipOn = Advice.OnNonDefaultValue.class)
+        static boolean skip() {
+            return true;
+        }
+
+        @Advice.OnMethodExit
+        static void exit(@Advice.This Inventory inventory,
+                @Advice.Return(readOnly = false) InventoryHolder holder) {
+            holder = inventory.getHolder();
+        }
+    }
+}

src/test/resources/META-INF/services/org.junit.platform.launcher.LauncherSessionListener

@@ -1 +1,2 @@
 net.tfminecraft.activitytf.testsupport.RegistryBootstrap
+net.tfminecraft.activitytf.testsupport.HolderLookupPatch