Fix Menu click handler using wrong slot offset on page 2+

codex-api/src/main/java/studio/magemonkey/codex/manager/api/menu/Menu.java

@@ -84,6 +84,26 @@ public Slot getSlot(int i) {
         return slots.get(i);
     }
 
+    /**
+     * Resolves a raw display slot (0..inventory size, as seen by the player/Bukkit)
+     * to the {@link Slot} bound to it on the currently open page. This applies the
+     * same page offset that {@link #open(int)} uses to render the page, so click
+     * handling and rendering can't drift out of sync.
+     */
+    @Nullable
+    public Slot getSlotOnCurrentPage(int displaySlot) {
+        return slots.get(toVirtualSlot(this.page, this.inventory.getSize(), displaySlot));
+    }
+
+    /**
+     * Maps a page-relative display slot to its virtual index in {@link #slots}.
+     * Shared by {@link #open(int)} (render) and {@link #getSlotOnCurrentPage(int)}
+     * (click) so the two can't disagree about what a display slot means.
+     */
+    static int toVirtualSlot(int page, int pageSize, int displaySlot) {
+        return page * pageSize + displaySlot;
+    }
+
     public void openSync() {
         new BukkitRunnable() {
             @Override
@@ -106,17 +126,16 @@ public void open(int page) {
         inventory = Bukkit.createInventory(Menu.this, rows * 9, title
                 .replace("%page%", String.valueOf(finalPage + 1))
                 .replace("%pages%", String.valueOf(getPages())));
+        Menu.this.page = finalPage;
         for (int i = 0, last = Menu.this.inventory.getSize(); i < last; i++) {
-            Slot slot = slots.get(finalPage * Menu.this.inventory.getSize() + i);
+            Slot slot = getSlotOnCurrentPage(i);
             if (slot != null) {
                 inventory.setItem(i, slot.getItemStack());
             }
         }
         Menu.this.opening = true;
         player.openInventory(inventory);
         Menu.this.opening = false;
-        Menu.this.page = finalPage;
-
     }
 
     public void openSubMenu(Menu menu) {

codex-api/src/main/java/studio/magemonkey/codex/manager/api/menu/MenuManager.java

@@ -50,7 +50,7 @@ public void onInventoryClick(InventoryClickEvent event) {
         InventoryHolder holder = inventory.getHolder();
         if (holder instanceof Menu) {
             event.setCancelled(true);
-            Slot slot = ((Menu) holder).getSlot(event.getSlot());
+            Slot slot = ((Menu) holder).getSlotOnCurrentPage(event.getSlot());
             if (slot != null) {
                 switch (event.getClick()) {
                     case LEFT -> slot.onLeftClick();

codex-api/src/test/java/studio/magemonkey/codex/manager/api/menu/MenuTest.java

@@ -0,0 +1,52 @@
+package studio.magemonkey.codex.manager.api.menu;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Covers the page-offset math shared by {@link Menu#open(int)} (render) and
+ * {@link Menu#getSlotOnCurrentPage(int)} (click resolution). Regression test for
+ * https://github.com/magemonkeystudio/codex/issues/156, where render and click
+ * disagreed about what a display slot meant on page 2+.
+ */
+class MenuTest {
+
+    @Test
+    void toVirtualSlot_firstPageIsUnshifted() {
+        assert Menu.toVirtualSlot(0, 54, 0) == 0;
+        assert Menu.toVirtualSlot(0, 54, 5) == 5;
+        assert Menu.toVirtualSlot(0, 54, 53) == 53;
+    }
+
+    @Test
+    void toVirtualSlot_laterPagesAreShiftedByPageSize() {
+        assert Menu.toVirtualSlot(1, 54, 0) == 54;
+        assert Menu.toVirtualSlot(1, 54, 5) == 59;
+        assert Menu.toVirtualSlot(2, 54, 5) == 113;
+    }
+
+    @Test
+    void toVirtualSlot_matchesAcrossRenderAndClickForSameDisplaySlot() {
+        // The exact scenario from #156: a display slot on page 2 must resolve to the
+        // same virtual slot whether it's being rendered (open) or clicked (MenuManager).
+        int pageSize    = 9;
+        int displaySlot = 3;
+        int page        = 2;
+
+        int renderVirtualSlot = Menu.toVirtualSlot(page, pageSize, displaySlot);
+        int clickVirtualSlot  = Menu.toVirtualSlot(page, pageSize, displaySlot);
+
+        assert renderVirtualSlot == clickVirtualSlot;
+        assert renderVirtualSlot == 21;
+    }
+
+    @Test
+    void toVirtualSlot_differentPagesDoNotCollideForSameDisplaySlot() {
+        int pageSize    = 9;
+        int displaySlot = 3;
+
+        int page1Slot = Menu.toVirtualSlot(0, pageSize, displaySlot);
+        int page2Slot = Menu.toVirtualSlot(1, pageSize, displaySlot);
+
+        assert page1Slot != page2Slot;
+    }
+}