Fix lecterns on 1.20.60, add support for virtual books (#4471)

* Fix lecterns on 1.20.60, start on virtual lecterns

* Fix: virtual books & actual books opening twice, resolve other issues, remove debug

* undo some unnecessary diff

* Don't try to send virtual books to pre 1.20.60 clients

* address review by camotoy

core/src/main/java/org/geysermc/geyser/inventory/holder/BlockInventoryHolder.java

@@ -32,8 +32,10 @@ import org.cloudburstmc.protocol.bedrock.packet.BlockEntityDataPacket;
 import org.cloudburstmc.protocol.bedrock.packet.ContainerClosePacket;
 import org.cloudburstmc.protocol.bedrock.packet.ContainerOpenPacket;
 import org.cloudburstmc.protocol.bedrock.packet.UpdateBlockPacket;
+import org.geysermc.geyser.GeyserImpl;
 import org.geysermc.geyser.inventory.Container;
 import org.geysermc.geyser.inventory.Inventory;
+import org.geysermc.geyser.inventory.LecternContainer;
 import org.geysermc.geyser.registry.BlockRegistries;
 import org.geysermc.geyser.registry.type.BlockMapping;
 import org.geysermc.geyser.session.GeyserSession;
@@ -151,13 +153,27 @@ public class BlockInventoryHolder extends InventoryHolder {
 
     @Override
     public void closeInventory(InventoryTranslator translator, GeyserSession session, Inventory inventory) {
-        if (((Container) inventory).isUsingRealBlock()) {
+        if (inventory instanceof Container container) {
-            // No need to reset a block since we didn't change any blocks
+            if (container.isUsingRealBlock() && !(inventory instanceof LecternContainer)) {
-            // But send a container close packet because we aren't destroying the original.
+                // No need to reset a block since we didn't change any blocks
-            ContainerClosePacket packet = new ContainerClosePacket();
+                // But send a container close packet because we aren't destroying the original.
-            packet.setId((byte) inventory.getBedrockId());
+                ContainerClosePacket packet = new ContainerClosePacket();
-            packet.setServerInitiated(true);
+                packet.setId((byte) inventory.getBedrockId());
-            session.sendUpstreamPacket(packet);
+                packet.setServerInitiated(true);
+                session.sendUpstreamPacket(packet);
+                return;
+            }
+        } else {
+            GeyserImpl.getInstance().getLogger().warning("Tried to close a non-container inventory in a block inventory holder! ");
+            if (GeyserImpl.getInstance().getLogger().isDebug()) {
+                GeyserImpl.getInstance().getLogger().debug("Current inventory: " + inventory);
+                GeyserImpl.getInstance().getLogger().debug("Open inventory: " + session.getOpenInventory());
+            }
+            // Try to save ourselves? maybe?
+            // https://github.com/GeyserMC/Geyser/issues/4141
+            // TODO: improve once this issue is pinned down properly
+            session.setOpenInventory(null);
+            session.setInventoryTranslator(InventoryTranslator.PLAYER_INVENTORY_TRANSLATOR);
             return;
         }
 

core/src/main/java/org/geysermc/geyser/inventory/item/StoredItemMappings.java

@@ -53,6 +53,7 @@ public class StoredItemMappings {
     private final ItemMapping upgradeTemplate;
     private final ItemMapping wheat;
     private final ItemMapping writableBook;
+    private final ItemMapping writtenBook;
 
     public StoredItemMappings(Map<Item, ItemMapping> itemMappings) {
         this.bamboo = load(itemMappings, Items.BAMBOO);
@@ -68,6 +69,7 @@ public class StoredItemMappings {
         this.upgradeTemplate = load(itemMappings, Items.NETHERITE_UPGRADE_SMITHING_TEMPLATE);
         this.wheat = load(itemMappings, Items.WHEAT);
         this.writableBook = load(itemMappings, Items.WRITABLE_BOOK);
+        this.writtenBook = load(itemMappings, Items.WRITTEN_BOOK);
     }
 
     @NonNull

core/src/main/java/org/geysermc/geyser/session/GeyserSession.java

@@ -89,6 +89,7 @@ import org.cloudburstmc.protocol.bedrock.data.command.CommandEnumData;
 import org.cloudburstmc.protocol.bedrock.data.command.CommandPermission;
 import org.cloudburstmc.protocol.bedrock.data.command.SoftEnumUpdateType;
 import org.cloudburstmc.protocol.bedrock.data.entity.EntityFlag;
+import org.cloudburstmc.protocol.bedrock.data.inventory.ItemData;
 import org.cloudburstmc.protocol.bedrock.packet.*;
 import org.cloudburstmc.protocol.common.DefinitionRegistry;
 import org.cloudburstmc.protocol.common.util.OptionalBoolean;
@@ -595,6 +596,12 @@ public class GeyserSession implements GeyserConnection, GeyserCommandSource {
      */
     private final Queue<Long> keepAliveCache = new ConcurrentLinkedQueue<>();
 
+    /**
+     * Stores the book that is currently being read. Used in {@link org.geysermc.geyser.translator.protocol.java.inventory.JavaOpenBookTranslator}
+     */
+    @Setter
+    private @Nullable ItemData currentBook = null;
+
     private final GeyserCameraData cameraData;
 
     private final GeyserEntityData entityData;

core/src/main/java/org/geysermc/geyser/translator/inventory/LecternInventoryTranslator.java

@@ -36,36 +36,71 @@ import org.cloudburstmc.nbt.NbtMapBuilder;
 import org.cloudburstmc.nbt.NbtType;
 import org.cloudburstmc.protocol.bedrock.data.inventory.ItemData;
 import org.geysermc.erosion.util.LecternUtils;
+import org.geysermc.geyser.GeyserImpl;
+import org.geysermc.geyser.inventory.Container;
 import org.geysermc.geyser.inventory.GeyserItemStack;
 import org.geysermc.geyser.inventory.Inventory;
 import org.geysermc.geyser.inventory.LecternContainer;
 import org.geysermc.geyser.inventory.PlayerInventory;
-import org.geysermc.geyser.inventory.updater.InventoryUpdater;
+import org.geysermc.geyser.inventory.updater.ContainerInventoryUpdater;
+import org.geysermc.geyser.network.GameProtocol;
 import org.geysermc.geyser.session.GeyserSession;
 import org.geysermc.geyser.util.BlockEntityUtils;
 import org.geysermc.geyser.util.InventoryUtils;
 
 import java.util.Collections;
 
-public class LecternInventoryTranslator extends BaseInventoryTranslator {
+public class LecternInventoryTranslator extends AbstractBlockInventoryTranslator {
-    private final InventoryUpdater updater;
+
+    /**
+     * Hack: Java opens a lectern first, and then follows it up with a ClientboundContainerSetContentPacket
+     * to actually send the book's contents. We delay opening the inventory until the book was sent.
+     */
+    private boolean initialized = false;
 
     public LecternInventoryTranslator() {
-        super(1);
+        super(1, "minecraft:lectern[facing=north,has_book=true,powered=true]", org.cloudburstmc.protocol.bedrock.data.inventory.ContainerType.LECTERN , ContainerInventoryUpdater.INSTANCE);
-        this.updater = new InventoryUpdater();
     }
 
     @Override
     public boolean prepareInventory(GeyserSession session, Inventory inventory) {
+        super.prepareInventory(session, inventory);
+        if (((Container) inventory).isUsingRealBlock()) {
+            initialized = false; // We have to wait until we get the book to show to the client
+        } else {
+            updateBook(session, inventory, inventory.getItem(0)); // See JavaOpenBookTranslator; placed here manually
+            initialized = true;
+        }
         return true;
     }
 
     @Override
     public void openInventory(GeyserSession session, Inventory inventory) {
+        // Hacky, but we're dealing with LECTERNS! It cannot not be hacky.
+        // "initialized" indicates whether we've received the book from the Java server yet.
+        // dropping lectern book is the fun workaround when we have to enter the gui to drop the book.
+        // Since we leave it immediately... don't open it!
+        if (initialized && !session.isDroppingLecternBook()) {
+            super.openInventory(session, inventory);
+        }
     }
 
     @Override
     public void closeInventory(GeyserSession session, Inventory inventory) {
+        // Of course, sending a simple ContainerClosePacket, or even breaking the block doesn't work to close a lectern.
+        // Heck, the latter crashes the client xd
+        // BDS just sends an empty base lectern tag... that kicks out the client. Fine. Let's do that!
+        LecternContainer lecternContainer = (LecternContainer) inventory;
+        Vector3i position = lecternContainer.isUsingRealBlock() ? session.getLastInteractionBlockPosition() : inventory.getHolderPosition();
+        var baseLecternTag = LecternUtils.getBaseLecternTag(position.getX(), position.getY(), position.getZ(), 0);
+        BlockEntityUtils.updateBlockEntity(session, baseLecternTag.build(), position);
+
+        super.closeInventory(session, inventory); // Removes the fake blocks if need be
+
+        // Now: Restore the lectern, if it actually exists
+        if (lecternContainer.isUsingRealBlock()) {
+            GeyserImpl.getInstance().getWorldManager().sendLecternData(session, position.getX(), position.getY(), position.getZ());
+        }
     }
 
     @Override
@@ -82,13 +117,19 @@ public class LecternInventoryTranslator extends BaseInventoryTranslator {
     public void updateInventory(GeyserSession session, Inventory inventory) {
         GeyserItemStack itemStack = inventory.getItem(0);
         if (!itemStack.isEmpty()) {
+            boolean isDropping = session.isDroppingLecternBook();
             updateBook(session, inventory, itemStack);
+
+            if (!initialized && !isDropping) {
+                initialized = true;
+                openInventory(session, inventory);
+            }
         }
     }
 
     @Override
     public void updateSlot(GeyserSession session, Inventory inventory, int slot) {
-        this.updater.updateSlot(this, session, inventory, slot);
+        super.updateSlot(session, inventory, slot);
         if (slot == 0) {
             updateBook(session, inventory, inventory.getItem(0));
         }
@@ -107,11 +148,14 @@ public class LecternInventoryTranslator extends BaseInventoryTranslator {
             InventoryUtils.closeInventory(session, inventory.getJavaId(), false);
         } else if (lecternContainer.getBlockEntityTag() == null) {
             CompoundTag tag = book.getNbt();
-            // Position has to be the last interacted position... right?
+            Vector3i position = lecternContainer.isUsingRealBlock() ? session.getLastInteractionBlockPosition() : inventory.getHolderPosition();
-            Vector3i position = session.getLastInteractionBlockPosition();
+
             // If shouldExpectLecternHandled returns true, this is already handled for us
             // shouldRefresh means that we should boot out the client on our side because their lectern GUI isn't updated yet
-            boolean shouldRefresh = !session.getGeyser().getWorldManager().shouldExpectLecternHandled(session) && !session.getLecternCache().contains(position);
+            // TODO: yeet after 1.20.60 is minimum supported version
+            boolean shouldRefresh = !session.getGeyser().getWorldManager().shouldExpectLecternHandled(session)
+                    && !session.getLecternCache().contains(position)
+                    && !GameProtocol.is1_20_60orHigher(session.getUpstream().getProtocolVersion());
 
             NbtMap blockEntityTag;
             if (tag != null) {
@@ -147,10 +191,11 @@ public class LecternInventoryTranslator extends BaseInventoryTranslator {
             // the block entity tag
             lecternContainer.setBlockEntityTag(blockEntityTag);
             lecternContainer.setPosition(position);
+
+            BlockEntityUtils.updateBlockEntity(session, blockEntityTag, position);
+            session.getLecternCache().add(position);
+
             if (shouldRefresh) {
-                // Update the lectern because it's not updated client-side
-                BlockEntityUtils.updateBlockEntity(session, blockEntityTag, position);
-                session.getLecternCache().add(position);
                 // Close the window - we will reopen it once the client has this data synced
                 ServerboundContainerClosePacket closeWindowPacket = new ServerboundContainerClosePacket(lecternContainer.getJavaId());
                 session.sendDownstreamGamePacket(closeWindowPacket);
@@ -161,6 +206,6 @@ public class LecternInventoryTranslator extends BaseInventoryTranslator {
 
     @Override
     public Inventory createInventory(String name, int windowId, ContainerType containerType, PlayerInventory playerInventory) {
-        return new LecternContainer(name, windowId, this.size, containerType, playerInventory);
+        return new LecternContainer(name, windowId, this.size + playerInventory.getSize(), containerType, playerInventory);
     }
 }

core/src/main/java/org/geysermc/geyser/translator/protocol/bedrock/BedrockInventoryTransactionTranslator.java

@@ -380,6 +380,8 @@ public class BedrockInventoryTransactionTranslator extends PacketTranslator<Inve
                             } else if (packet.getItemInHand().getDefinition() == session.getItemMappings().getStoredItems().glassBottle().getBedrockDefinition()) {
                                 // Handled in case 0
                                 break;
+                            } else if (packet.getItemInHand().getDefinition() == session.getItemMappings().getStoredItems().writtenBook().getBedrockDefinition()) {
+                                session.setCurrentBook(packet.getItemInHand());
                             }
                         }
 
@@ -387,7 +389,7 @@ public class BedrockInventoryTransactionTranslator extends PacketTranslator<Inve
                         session.sendDownstreamGamePacket(useItemPacket);
 
                         List<LegacySetItemSlotData> legacySlots = packet.getLegacySlots();
-                        if (packet.getActions().size() == 1 && legacySlots.size() > 0) {
+                        if (packet.getActions().size() == 1 && !legacySlots.isEmpty()) {
                             InventoryActionData actionData = packet.getActions().get(0);
                             LegacySetItemSlotData slotData = legacySlots.get(0);
                             if (slotData.getContainerId() == 6 && !actionData.getFromItem().isNull()) {

core/src/main/java/org/geysermc/geyser/translator/protocol/bedrock/BedrockLecternUpdateTranslator.java

@@ -31,8 +31,10 @@ import com.github.steveice10.mc.protocol.packet.ingame.serverbound.inventory.Ser
 import com.github.steveice10.mc.protocol.packet.ingame.serverbound.inventory.ServerboundContainerClosePacket;
 import com.github.steveice10.mc.protocol.packet.ingame.serverbound.player.ServerboundUseItemOnPacket;
 import org.cloudburstmc.protocol.bedrock.packet.LecternUpdatePacket;
+import org.geysermc.geyser.inventory.Inventory;
 import org.geysermc.geyser.inventory.LecternContainer;
 import org.geysermc.geyser.session.GeyserSession;
+import org.geysermc.geyser.translator.inventory.LecternInventoryTranslator;
 import org.geysermc.geyser.translator.protocol.PacketTranslator;
 import org.geysermc.geyser.translator.protocol.Translator;
 import org.geysermc.geyser.util.InventoryUtils;
@@ -77,6 +79,15 @@ public class BedrockLecternUpdateTranslator extends PacketTranslator<LecternUpda
                 int newJavaPage = (packet.getPage() * 2);
                 int currentJavaPage = (lecternContainer.getCurrentBedrockPage() * 2);
 
+                // So, fun fact: We need to separately handle fake lecterns!
+                // Since those are not actually a real lectern... the Java server won't respond to our requests.
+                if (!lecternContainer.isUsingRealBlock()) {
+                    LecternInventoryTranslator translator = (LecternInventoryTranslator) session.getInventoryTranslator();
+                    Inventory inventory = session.getOpenInventory();
+                    translator.updateProperty(session, inventory, 0, newJavaPage);
+                    return;
+                }
+
                 // Send as many click button packets as we need to
                 // Java has the option to specify exact page numbers by adding 100 to the number, but buttonId variable
                 // is a byte when transmitted over the network and therefore this stops us at 128

core/src/main/java/org/geysermc/geyser/translator/protocol/bedrock/entity/player/BedrockInteractTranslator.java

@@ -43,6 +43,7 @@ import org.geysermc.geyser.item.Items;
 import org.geysermc.geyser.session.GeyserSession;
 import org.geysermc.geyser.translator.protocol.PacketTranslator;
 import org.geysermc.geyser.translator.protocol.Translator;
+import org.geysermc.geyser.util.InventoryUtils;
 
 import java.util.concurrent.TimeUnit;
 
@@ -134,6 +135,10 @@ public class BedrockInteractTranslator extends PacketTranslator<InteractPacket>
                         containerOpenPacket.setBlockPosition(entity.getPosition().toInt());
                         session.sendUpstreamPacket(containerOpenPacket);
                     }
+                } else {
+                    // Case: Player opens a player inventory, while we think it shouldn't have!
+                    // Close all inventories, reset to player inventory.
+                    InventoryUtils.closeInventory(session, session.getOpenInventory().getJavaId(), false);
                 }
                 break;
         }

core/src/main/java/org/geysermc/geyser/translator/protocol/java/inventory/JavaContainerSetContentTranslator.java

@@ -27,6 +27,7 @@ package org.geysermc.geyser.translator.protocol.java.inventory;
 
 import com.github.steveice10.mc.protocol.packet.ingame.clientbound.inventory.ClientboundContainerSetContentPacket;
 import org.geysermc.geyser.GeyserImpl;
+import org.geysermc.geyser.GeyserLogger;
 import org.geysermc.geyser.inventory.GeyserItemStack;
 import org.geysermc.geyser.inventory.Inventory;
 import org.geysermc.geyser.session.GeyserSession;
@@ -48,12 +49,12 @@ public class JavaContainerSetContentTranslator extends PacketTranslator<Clientbo
         int inventorySize = inventory.getSize();
         for (int i = 0; i < packet.getItems().length; i++) {
             if (i >= inventorySize) {
-                GeyserImpl geyser = session.getGeyser();
+                GeyserLogger logger = session.getGeyser().getLogger();
-                geyser.getLogger().warning("ClientboundContainerSetContentPacket sent to " + session.bedrockUsername()
+                logger.warning("ClientboundContainerSetContentPacket sent to " + session.bedrockUsername()
                         + " that exceeds inventory size!");
-                if (geyser.getConfig().isDebugMode()) {
+                if (logger.isDebug()) {
-                    geyser.getLogger().debug(packet);
+                    logger.debug(packet);
-                    geyser.getLogger().debug(inventory);
+                    logger.debug(inventory);
                 }
                 updateInventory(session, inventory, packet.getContainerId());
                 // 1.18.1 behavior: the previous items will be correctly set, but the state ID and carried item will not

core/src/main/java/org/geysermc/geyser/translator/protocol/java/inventory/JavaContainerSetSlotTranslator.java

@@ -34,7 +34,7 @@ import org.cloudburstmc.protocol.bedrock.data.inventory.crafting.recipe.ShapedRe
 import org.cloudburstmc.protocol.bedrock.data.inventory.descriptor.ItemDescriptorWithCount;
 import org.cloudburstmc.protocol.bedrock.packet.CraftingDataPacket;
 import org.cloudburstmc.protocol.bedrock.packet.InventorySlotPacket;
-import org.geysermc.geyser.GeyserImpl;
+import org.geysermc.geyser.GeyserLogger;
 import org.geysermc.geyser.inventory.GeyserItemStack;
 import org.geysermc.geyser.inventory.Inventory;
 import org.geysermc.geyser.inventory.recipe.GeyserShapedRecipe;
@@ -65,8 +65,9 @@ public class JavaContainerSetSlotTranslator extends PacketTranslator<Clientbound
 
         //TODO: support window id -2, should update player inventory
         Inventory inventory = InventoryUtils.getInventory(session, packet.getContainerId());
-        if (inventory == null)
+        if (inventory == null) {
             return;
+        }
 
         InventoryTranslator translator = session.getInventoryTranslator();
         if (translator != null) {
@@ -76,12 +77,12 @@ public class JavaContainerSetSlotTranslator extends PacketTranslator<Clientbound
 
             int slot = packet.getSlot();
             if (slot >= inventory.getSize()) {
-                GeyserImpl geyser = session.getGeyser();
+                GeyserLogger logger = session.getGeyser().getLogger();
-                geyser.getLogger().warning("ClientboundContainerSetSlotPacket sent to " + session.bedrockUsername()
+                logger.warning("ClientboundContainerSetSlotPacket sent to " + session.bedrockUsername()
                         + " that exceeds inventory size!");
-                if (geyser.getConfig().isDebugMode()) {
+                if (logger.isDebug()) {
-                    geyser.getLogger().debug(packet);
+                    logger.debug(packet.toString());
-                    geyser.getLogger().debug(inventory);
+                    logger.debug(inventory.toString());
                 }
                 // 1.19.0 behavior: the state ID will not be set due to exception
                 return;

core/src/main/java/org/geysermc/geyser/translator/protocol/java/inventory/JavaOpenBookTranslator.java

@@ -0,0 +1,87 @@
+/*
+ * Copyright (c) 2019-2024 GeyserMC. http://geysermc.org
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ *
+ * @author GeyserMC
+ * @link https://github.com/GeyserMC/Geyser
+ */
+
+package org.geysermc.geyser.translator.protocol.java.inventory;
+
+import com.github.steveice10.mc.protocol.data.game.inventory.ContainerType;
+import com.github.steveice10.mc.protocol.packet.ingame.clientbound.inventory.ClientboundOpenBookPacket;
+import com.github.steveice10.mc.protocol.packet.ingame.serverbound.inventory.ServerboundContainerClosePacket;
+import org.geysermc.geyser.inventory.GeyserItemStack;
+import org.geysermc.geyser.inventory.Inventory;
+import org.geysermc.geyser.item.Items;
+import org.geysermc.geyser.network.GameProtocol;
+import org.geysermc.geyser.session.GeyserSession;
+import org.geysermc.geyser.translator.inventory.InventoryTranslator;
+import org.geysermc.geyser.translator.protocol.PacketTranslator;
+import org.geysermc.geyser.translator.protocol.Translator;
+import org.geysermc.geyser.util.InventoryUtils;
+
+import java.util.Objects;
+
+@Translator(packet = ClientboundOpenBookPacket.class)
+public class JavaOpenBookTranslator extends PacketTranslator<ClientboundOpenBookPacket> {
+
+    /**
+     * Unlike other fake inventories that rely on placing blocks in the world;
+     * the virtual lectern workaround for books isn't triggered the same way.
+     * Specifically, we don't get a window id - hence, we just use our own!
+     */
+    private final static int FAKE_LECTERN_WINDOW_ID = -69;
+
+    @Override
+    public void translate(GeyserSession session, ClientboundOpenBookPacket packet) {
+        GeyserItemStack stack = session.getPlayerInventory().getItemInHand();
+
+        // Don't spawn a fake lectern for books already opened "normally" by the client.
+        if (stack.getItemData(session).equals(session.getCurrentBook())) {
+            session.setCurrentBook(null);
+            return;
+        }
+
+        // Only post 1.20.60 is it possible to tell the client to open a lectern.
+        if (!GameProtocol.is1_20_60orHigher(session.getUpstream().getProtocolVersion())) {
+            return;
+        }
+
+        if (stack.asItem().equals(Items.WRITTEN_BOOK)) {
+            Inventory openInventory = session.getOpenInventory();
+            if (openInventory != null) {
+                InventoryUtils.closeInventory(session, openInventory.getJavaId(), true);
+
+                ServerboundContainerClosePacket closeWindowPacket = new ServerboundContainerClosePacket(openInventory.getJavaId());
+                session.sendDownstreamGamePacket(closeWindowPacket);
+            }
+
+            InventoryTranslator translator = InventoryTranslator.inventoryTranslator(ContainerType.LECTERN);
+            session.setInventoryTranslator(translator);
+
+            // Should never be null
+            Objects.requireNonNull(translator, "lectern translator must exist");
+            Inventory inventory = translator.createInventory("", FAKE_LECTERN_WINDOW_ID, ContainerType.LECTERN , session.getPlayerInventory());
+            inventory.setItem(0, stack, session);
+            InventoryUtils.openInventory(session, inventory);
+        }
+    }
+}

core/src/main/java/org/geysermc/geyser/util/InventoryUtils.java

@@ -45,6 +45,7 @@ import org.geysermc.geyser.GeyserImpl;
 import org.geysermc.geyser.inventory.Container;
 import org.geysermc.geyser.inventory.GeyserItemStack;
 import org.geysermc.geyser.inventory.Inventory;
+import org.geysermc.geyser.inventory.LecternContainer;
 import org.geysermc.geyser.inventory.PlayerInventory;
 import org.geysermc.geyser.inventory.click.Click;
 import org.geysermc.geyser.inventory.recipe.GeyserRecipe;
@@ -123,7 +124,9 @@ public class InventoryUtils {
         if (inventory != null) {
             InventoryTranslator translator = session.getInventoryTranslator();
             translator.closeInventory(session, inventory);
-            if (confirm && inventory.isDisplayed() && !inventory.isPending() && !(translator instanceof LecternInventoryTranslator)) {
+            if (confirm && inventory.isDisplayed() && !inventory.isPending()
+                    && !(translator instanceof LecternInventoryTranslator) // TODO: double-check
+            ) {
                 session.setClosingInventory(true);
             }
         }
@@ -133,6 +136,10 @@ public class InventoryUtils {
 
     public static @Nullable Inventory getInventory(GeyserSession session, int javaId) {
         if (javaId == 0) {
+            // ugly hack: lecterns aren't their own inventory on Java, and can hence be closed with e.g. an id of 0
+            if (session.getOpenInventory() instanceof LecternContainer) {
+                return session.getOpenInventory();
+            }
             return session.getPlayerInventory();
         } else {
             Inventory openInventory = session.getOpenInventory();