Add /hullhider to BauSystem

/hullhider hides the inside of the build area hull of a region from the
players who toggle it, like the hull hider of a fight. Each region gets its
own hull while somebody uses it; it is rebuilt every time somebody enables it,
as WorldEdit and Axiom changes do not update it. /techhider, /xray and
/hullhider exclude each other per player and region.

Which players may see the hull contents is now decided by Viewers, a sealed
class with HiddenByDefault (fight) and VisibleByDefault (bau). Added players
are stored in a set; isViewer alone defines what being added means.

BauSystem/BauSystem_Main/src/BauSystem.properties

@@ -37,6 +37,7 @@ SCOREBOARD_TPS_FROZEN = §eFrozen
 SCOREBOARD_TRACE_TICKS = Ticks
 SCOREBOARD_TECHHIDER = TechHider§8: §aOn
 SCOREBOARD_XRAY = XRay§8: §aOn
+SCOREBOARD_HULLHIDER = HullHider§8: §aOn
 SCOREBOARD_LOCK_TEAM = Bau Lock§8: §eTeam
 SCOREBOARD_LOCK_SUPERVISOR = Bau Lock§8: §eSupervisor
 SCOREBOARD_LOCK_TEAM_AND_SERVERTEAM = Bau Lock§8: §e(Server) Team
@@ -1033,6 +1034,11 @@ XRAY_HELP = §8/§exray §8- §7Toggle Xray
 XRAY_GLOBAL = §cNo xray in global region
 XRAY_ON = §aXray activated
 XRAY_OFF = §cXray deactivated
+# HULLHIDER
+HULLHIDER_HELP = §8/§ehullhider §8- §7Toggle Hullhider
+HULLHIDER_GLOBAL = §cNo hullhider in global region
+HULLHIDER_ON = §aHullhider activated
+HULLHIDER_OFF = §cHullhider deactivated
 # WorldEdit
 COLORREPLACE_HELP = §8//§ecolorreplace §8[§7color§8] §8[§7color§8] §8- §7Replace all blocks of one color with another
 TYPEREPLACE_HELP = §8//§etypereplace §8[§7type§8] §8[§7type§8] §8- §7Replace all blocks of one type with another

BauSystem/BauSystem_Main/src/BauSystem_de.properties

@@ -37,6 +37,7 @@ SCOREBOARD_TPS_FROZEN = §eEingefroren
 SCOREBOARD_TRACE_TICKS = Ticks
 SCOREBOARD_TECHHIDER = TechHider§8: §aAn
 SCOREBOARD_XRAY = XRay§8: §aAn
+SCOREBOARD_HULLHIDER = HullHider§8: §aAn
 SCOREBOARD_LOCK_TEAM = Bau Lock§8: §eTeam
 SCOREBOARD_LOCK_TEAM_AND_SERVERTEAM = Bau Lock§8: §e(Server-) Team
 SCOREBOARD_LOCK_SERVERTEAM = Bau Lock§8: §eServerteam
@@ -966,6 +967,11 @@ XRAY_HELP = §8/§exray §8- §7Xray umschalten
 XRAY_GLOBAL = §cKein Xray in der globalen region
 XRAY_ON = §aXray aktiviert
 XRAY_OFF = §cXray deaktiviert
+# HULLHIDER
+HULLHIDER_HELP = §8/§ehullhider §8- §7Hullhider umschalten
+HULLHIDER_GLOBAL = §cKein Hullhider in der globalen region
+HULLHIDER_ON = §aHullhider aktiviert
+HULLHIDER_OFF = §cHullhider deaktiviert
 # WorldEdit
 COLORREPLACE_HELP = §8//§ecolorreplace §8[§7color§8] §8[§7color§8] §8- §7Ersetzt eine Farbe mit einer anderen
 TYPEREPLACE_HELP = §8//§etyreplace §8[§7type§8] §8[§7type§8] §8- §7Ersetzt einen Blockgruppe mit einer anderen

BauSystem/BauSystem_Main/src/de/steamwar/bausystem/features/hullhider/HullHiderCommand.java

@@ -0,0 +1,179 @@
+/*
+ *   This file is a part of the SteamWar software.
+ *
+ *   Copyright (C) 2026 SteamWar.de-Serverteam
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU Affero General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ *   GNU Affero General Public License for more details.
+ *
+ *   You should have received a copy of the GNU Affero General Public License
+ *   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package de.steamwar.bausystem.features.hullhider;
+
+import de.steamwar.bausystem.BauSystem;
+import de.steamwar.bausystem.features.techhider.HiddenHull;
+import de.steamwar.bausystem.features.techhider.TechHiderCommand;
+import de.steamwar.bausystem.features.xray.XrayCommand;
+import de.steamwar.bausystem.region.Point;
+import de.steamwar.bausystem.region.Region;
+import de.steamwar.bausystem.utils.ScoreboardElement;
+import de.steamwar.command.SWCommand;
+import de.steamwar.hullhider.Hull;
+import de.steamwar.hullhider.HullHider;
+import de.steamwar.hullhider.HullRegion;
+import de.steamwar.hullhider.Viewers;
+import de.steamwar.linkage.Linked;
+import de.steamwar.linkage.LinkedInstance;
+import de.steamwar.techhider.ObfuscationConfig;
+import net.md_5.bungee.api.ChatMessageType;
+import org.bukkit.Bukkit;
+import org.bukkit.World;
+import org.bukkit.entity.Entity;
+import org.bukkit.entity.Player;
+import org.bukkit.event.EventHandler;
+import org.bukkit.event.Listener;
+import org.bukkit.event.player.PlayerQuitEvent;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+
+/**
+ * Hides the inside of the build area hull of a region from the players who toggle it, as the hull hider of a fight would.
+ */
+@Linked
+public class HullHiderCommand extends SWCommand implements Listener, ScoreboardElement {
+
+    public HullHiderCommand() {
+        super("hullhider");
+
+        Bukkit.getPluginManager().registerEvents(hullHider, BauSystem.getInstance());
+        Bukkit.getScheduler().runTaskTimer(BauSystem.getInstance(), this::onTick, 1, 1);
+    }
+
+    private final HullHider<Region> hullHider = new HullHider<>();
+    // Read on netty threads by the BauTechHider
+    private final Map<Region, HiddenHull> hulls = new ConcurrentHashMap<>();
+    private final Map<Region, Set<Player>> hidden = new ConcurrentHashMap<>();
+
+    @LinkedInstance
+    public TechHiderCommand techHiderCommand;
+
+    @LinkedInstance
+    public XrayCommand xrayCommand;
+
+    @Register(description = "HULLHIDER_HELP")
+    public void toggleHider(@Validator Player player) {
+        Region region = Region.getRegion(player.getLocation());
+        if (region.getType().isGlobal()) {
+            BauSystem.MESSAGE.send("HULLHIDER_GLOBAL", player);
+            return;
+        }
+        if (!region.getGameModeConfig().Techhider.Active) {
+            return;
+        }
+
+        techHiderCommand.disable(region, player);
+        xrayCommand.disable(region, player);
+        if (hidden.getOrDefault(region, Collections.emptySet()).contains(player)) {
+            disable(region, player);
+            BauSystem.MESSAGE.sendPrefixless("HULLHIDER_OFF", player, ChatMessageType.ACTION_BAR);
+        } else {
+            enable(region, player);
+            BauSystem.MESSAGE.sendPrefixless("HULLHIDER_ON", player, ChatMessageType.ACTION_BAR);
+        }
+    }
+
+    private void enable(Region region, Player player) {
+        if (!hulls.containsKey(region)) {
+            Point min = region.getBuildArea().getMinPoint(true);
+            Point max = region.getBuildArea().getMaxPoint(true);
+            HullRegion hullRegion = HullRegion.fromInclusive(min.getX(), min.getY(), min.getZ(), max.getX(), max.getY(), max.getZ());
+
+            // Only the hidden blocks of an active techhider occlude, the hull hider is used without one
+            Hull hull = new Hull(BauSystem.getInstance(), world(), hullRegion, false, Collections.emptySet(), new Viewers.VisibleByDefault());
+            hullHider.addHull(region, hull);
+            hulls.put(region, new HiddenHull(hull, ObfuscationConfig.of(region.getGameModeConfig().Techhider).getObfuscationState()));
+        }
+
+        hidden.computeIfAbsent(region, rg -> ConcurrentHashMap.newKeySet()).add(player);
+        // Added players see the hull blocks of a hull visible by default
+        hullHider.addViewer(player, region);
+        // Block changes by WorldEdit or Axiom do not update the hull, so it is rebuilt every time somebody enables it.
+        // This also resends the chunks to the player.
+        hullHider.initialize(region);
+    }
+
+    /**
+     * Stops hiding the hull of the region from the player, removing the hull if nobody uses it anymore.
+     */
+    public void disable(Region region, Player player) {
+        Set<Player> players = hidden.get(region);
+        if (players == null || !players.remove(player)) return;
+
+        if (players.isEmpty()) {
+            hidden.remove(region);
+            hulls.remove(region);
+            hullHider.removeHull(region);
+        } else {
+            hullHider.removeViewer(player, region);
+            hullHider.resendChunks(player, region);
+        }
+    }
+
+    public void addHiddenHulls(Player player, List<HiddenHull> hiddenHulls) {
+        hidden.forEach((region, players) -> {
+            HiddenHull hiddenHull = hulls.get(region);
+            if (hiddenHull != null && players.contains(player)) {
+                hiddenHulls.add(hiddenHull);
+            }
+        });
+    }
+
+    private void onTick() {
+        if (hidden.isEmpty()) return;
+
+        for (Entity entity : world().getEntities()) {
+            hullHider.checkEntity(entity);
+        }
+        hullHider.sendUncoveredBlocks();
+    }
+
+    private World world() {
+        return Bukkit.getWorlds().get(0);
+    }
+
+    @EventHandler
+    public void onPlayerQuit(PlayerQuitEvent event) {
+        for (Region region : hidden.keySet()) {
+            disable(region, event.getPlayer());
+        }
+    }
+
+    @Override
+    public ScoreboardGroup getGroup() {
+        return ScoreboardGroup.OTHER;
+    }
+
+    @Override
+    public int order() {
+        return 0;
+    }
+
+    @Override
+    public String get(Region region, Player p) {
+        if (!hidden.getOrDefault(region, Collections.emptySet()).contains(p)) return null;
+        return "§e" + BauSystem.MESSAGE.parse("SCOREBOARD_HULLHIDER", p);
+    }
+}

BauSystem/BauSystem_Main/src/de/steamwar/bausystem/features/techhider/BauTechHider.java

@@ -19,6 +19,7 @@
 
 package de.steamwar.bausystem.features.techhider;
 
+import de.steamwar.bausystem.features.hullhider.HullHiderCommand;
 import de.steamwar.bausystem.features.world.SpectatorListener;
 import de.steamwar.bausystem.features.xray.XrayCommand;
 import de.steamwar.linkage.Linked;
@@ -29,18 +30,20 @@ import net.minecraft.world.level.block.Block;
 import net.minecraft.world.level.block.Blocks;
 import net.minecraft.world.level.block.entity.BlockEntityType;
 import net.minecraft.world.level.block.state.BlockState;
+import org.bukkit.craftbukkit.CraftWorld;
 import org.bukkit.entity.Player;
 
 import java.util.ArrayList;
 import java.util.List;
 
 /**
- * The single TechHider of the server. It combines the hiding rules of the spectator techhider, /techhider and /xray per player.
+ * The single TechHider of the server. It combines the hiding rules of the spectator techhider, /techhider and /xray and the hulls of /hullhider per player.
  */
 @Linked
 public class BauTechHider implements AccessPrivilegeProvider {
 
     private static final HidingRule[] NO_RULES = new HidingRule[0];
+    private static final HiddenHull[] NO_HULLS = new HiddenHull[0];
 
     // Injected after construction, until then nothing is hidden
     @LinkedInstance
@@ -49,6 +52,8 @@ public class BauTechHider implements AccessPrivilegeProvider {
     public TechHiderCommand techHiderCommand;
     @LinkedInstance
     public SpectatorListener spectatorListener;
+    @LinkedInstance
+    public HullHiderCommand hullHiderCommand;
 
     public BauTechHider() {
         new TechHider(this);
@@ -56,8 +61,8 @@ public class BauTechHider implements AccessPrivilegeProvider {
 
     @Override
     public ForPlayer forPlayer(Player p) {
-        if (xrayCommand == null || techHiderCommand == null || spectatorListener == null) {
-            return new BauPlayerAccess(p, NO_RULES);
+        if (xrayCommand == null || techHiderCommand == null || spectatorListener == null || hullHiderCommand == null) {
+            return new BauPlayerAccess(p, NO_RULES, NO_HULLS);
         }
 
         // Region rules first: where rules overlap, the first one decides the obfuscation block
@@ -65,7 +70,10 @@ public class BauTechHider implements AccessPrivilegeProvider {
         xrayCommand.addHidingRules(p, rules);
         techHiderCommand.addHidingRules(p, rules);
         spectatorListener.addHidingRules(p, rules);
-        return new BauPlayerAccess(p, rules.toArray(NO_RULES));
+
+        List<HiddenHull> hulls = new ArrayList<>();
+        hullHiderCommand.addHiddenHulls(p, hulls);
+        return new BauPlayerAccess(p, rules.toArray(NO_RULES), hulls.toArray(NO_HULLS));
     }
 
     @Override
@@ -76,7 +84,7 @@ public class BauTechHider implements AccessPrivilegeProvider {
     /**
      * Snapshot of the hiding rules of one player, taken once per outgoing packet on the netty thread.
      */
-    private record BauPlayerAccess(Player player, HidingRule[] rules) implements ForPlayer {
+    private record BauPlayerAccess(Player player, HidingRule[] rules, HiddenHull[] hulls) implements ForPlayer {
 
         @Override
         public Player getPlayer() {
@@ -88,11 +96,17 @@ public class BauTechHider implements AccessPrivilegeProvider {
             for (HidingRule rule : rules) {
                 if (rule.coversChunk(chunkX, chunkZ)) return false;
             }
+            for (HiddenHull hull : hulls) {
+                if (!hull.hull().getRegion().chunkOutside(chunkX, chunkZ)) return false;
+            }
             return true;
         }
 
         @Override
         public boolean canAccessPosition(int blockX, int blockY, int blockZ) {
+            for (HiddenHull hull : hulls) {
+                if (hull.hull().isBlockHidden(player, blockX, blockY, blockZ)) return false;
+            }
             return true;
         }
 
@@ -114,7 +128,10 @@ public class BauTechHider implements AccessPrivilegeProvider {
 
         @Override
         public boolean canAccessEntity(int entityId) {
-            return true;
+            if (hulls.length == 0) return true;
+
+            net.minecraft.world.entity.Entity nmsEntity = ((CraftWorld) player.getWorld()).getHandle().moonrise$getEntityLookup().get(entityId);
+            return nmsEntity == null || canAccessPosition(nmsEntity.getBlockX(), nmsEntity.getBlockY(), nmsEntity.getBlockZ());
         }
 
         @Override
@@ -130,7 +147,10 @@ public class BauTechHider implements AccessPrivilegeProvider {
             for (HidingRule rule : rules) {
                 if (rule.covers(blockX, blockY, blockZ)) return rule.obfuscation().getObfuscationState();
             }
-            // Only asked for positions a rule hides
+            for (HiddenHull hull : hulls) {
+                if (hull.hull().getRegion().contains(blockX, blockY, blockZ)) return hull.obfuscation();
+            }
+            // Only asked for positions a rule or hull hides
             return Blocks.END_STONE.defaultBlockState();
         }
     }

BauSystem/BauSystem_Main/src/de/steamwar/bausystem/features/techhider/HiddenHull.java

@@ -0,0 +1,29 @@
+/*
+ *   This file is a part of the SteamWar software.
+ *
+ *   Copyright (C) 2026 SteamWar.de-Serverteam
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU Affero General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ *   GNU Affero General Public License for more details.
+ *
+ *   You should have received a copy of the GNU Affero General Public License
+ *   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package de.steamwar.bausystem.features.techhider;
+
+import de.steamwar.hullhider.Hull;
+import net.minecraft.world.level.block.state.BlockState;
+
+/**
+ * A hull a player may not view, and the block sent instead of its hidden blocks.
+ */
+public record HiddenHull(Hull hull, BlockState obfuscation) {
+}

BauSystem/BauSystem_Main/src/de/steamwar/bausystem/features/techhider/TechHiderCommand.java

@@ -21,6 +21,7 @@ package de.steamwar.bausystem.features.techhider;
 
 import de.steamwar.bausystem.BauSystem;
 import de.steamwar.bausystem.Permission;
+import de.steamwar.bausystem.features.hullhider.HullHiderCommand;
 import de.steamwar.bausystem.features.world.SpectatorListener;
 import de.steamwar.bausystem.features.xray.XrayCommand;
 import de.steamwar.bausystem.region.Region;
@@ -58,6 +59,9 @@ public class TechHiderCommand extends SWCommand implements Listener, ScoreboardE
     @LinkedInstance
     public XrayCommand xrayCommand;
 
+    @LinkedInstance
+    public HullHiderCommand hullHiderCommand;
+
     @Register(description = "TECHHIDER_HELP")
     public void toggleHider(@Validator Player player) {
         Region region = Region.getRegion(player.getLocation());
@@ -79,6 +83,7 @@ public class TechHiderCommand extends SWCommand implements Listener, ScoreboardE
         }
 
         xrayCommand.disable(region, player);
+        hullHiderCommand.disable(region, player);
         if (hidden.get(region).contains(player)) {
             hidden.get(region).remove(player);
             BauSystem.MESSAGE.sendPrefixless("TECHHIDER_OFF", player, ChatMessageType.ACTION_BAR);

BauSystem/BauSystem_Main/src/de/steamwar/bausystem/features/xray/XrayCommand.java

@@ -21,6 +21,7 @@ package de.steamwar.bausystem.features.xray;
 
 import com.comphenix.tinyprotocol.TinyProtocol;
 import de.steamwar.bausystem.BauSystem;
+import de.steamwar.bausystem.features.hullhider.HullHiderCommand;
 import de.steamwar.bausystem.features.techhider.HidingRule;
 import de.steamwar.bausystem.features.techhider.TechHiderCommand;
 import de.steamwar.bausystem.region.Region;
@@ -62,6 +63,9 @@ public class XrayCommand extends SWCommand implements Listener, ScoreboardElemen
     @LinkedInstance
     public TechHiderCommand techHiderCommand;
 
+    @LinkedInstance
+    public HullHiderCommand hullHiderCommand;
+
     @Register(description = "XRAY_HELP")
     public void toggleHider(@Validator Player player) {
         Region region = Region.getRegion(player.getLocation());
@@ -87,6 +91,7 @@ public class XrayCommand extends SWCommand implements Listener, ScoreboardElemen
         }
 
         techHiderCommand.disable(region, player);
+        hullHiderCommand.disable(region, player);
         if (hidden.get(region).contains(player)) {
             hidden.get(region).remove(player);
             BauSystem.MESSAGE.sendPrefixless("XRAY_OFF", player, ChatMessageType.ACTION_BAR);

FightSystem/FightSystem_Core/src/de/steamwar/fightsystem/utils/FightHullHider.java

@@ -32,6 +32,7 @@ import de.steamwar.fightsystem.states.StateDependentListener;
 import de.steamwar.fightsystem.states.StateDependentTask;
 import de.steamwar.hullhider.Hull;
 import de.steamwar.hullhider.HullHider;
+import de.steamwar.hullhider.Viewers;
 import org.bukkit.Material;
 import org.bukkit.block.Block;
 import org.bukkit.entity.Player;
@@ -56,7 +57,7 @@ public class FightHullHider {
 
         Fight.teams().forEach(team -> {
             Region region = team.getSchemRegion();
-            hullHider.addHull(team, new Hull(FightSystem.getPlugin(), Config.world, region.toHullRegion(), region.getMinY() != Config.PlayerRegion.getMinY(), Config.GameModeConfig.Techhider.HiddenBlocks));
+            hullHider.addHull(team, new Hull(FightSystem.getPlugin(), Config.world, region.toHullRegion(), region.getMinY() != Config.PlayerRegion.getMinY(), Config.GameModeConfig.Techhider.HiddenBlocks, new Viewers.HiddenByDefault()));
         });
 
         new StateDependentListener(TechHiderWrapper.ENABLED, FightState.All, new MembershipListener());
@@ -109,7 +110,13 @@ public class FightHullHider {
 
         FightTeam team = Fight.getPlayerTeam(player);
         FightPlayer fp = Fight.getFightPlayer(player);
-        hullHider.forEach((hullTeam, hull) -> hullHider.setViewer(player, hullTeam, Config.isReferee(player) || hullTeam == team || (fp != null && fp.canEntern())));
+        hullHider.forEach((hullTeam, hull) -> {
+            if (Config.isReferee(player) || hullTeam == team || (fp != null && fp.canEntern())) {
+                hullHider.addViewer(player, hullTeam);
+            } else {
+                hullHider.removeViewer(player, hullTeam);
+            }
+        });
     }
 
     public void blockUpdate(Block block, Material changedType) {

HiderCore/src/de/steamwar/hullhider/Hull.java

@@ -45,7 +45,6 @@ import org.bukkit.entity.Player;
 import org.bukkit.plugin.Plugin;
 
 import java.util.*;
-import java.util.concurrent.ConcurrentHashMap;
 import java.util.function.BiConsumer;
 import java.util.logging.Level;
 import java.util.stream.IntStream;
@@ -67,21 +66,22 @@ public class Hull {
     private final HashMap<IntVector, BitSet> visibilityDirections = new HashMap<>(); // Contains the visible directions of each occluding visible block
     private final HashSet<IntVector> uncoveredSurface = new HashSet<>();
 
-    // Players that may see the hull contents (viewers). Everybody else is hidden from it (default deny). Concurrent as it is read on netty threads.
-    private final Set<UUID> viewers = ConcurrentHashMap.newKeySet();
+    private final Viewers viewers;
     private final HashSet<Entity> entities = new HashSet<>();
     private final HashSet<REntity> rentities = new HashSet<>();
 
     /**
      * @param groundVisible whether the bottom layer of the region is uncovered (if the region does not reach the ground)
      * @param hiddenBlocks  blocks that occlude like solid blocks, as the techhider replaces them by an occluding block
+     * @param viewers       which players may see the hull contents, a new instance for every hull
      */
-    public Hull(Plugin plugin, World world, HullRegion region, boolean groundVisible, Set<Material> hiddenBlocks) {
+    public Hull(Plugin plugin, World world, HullRegion region, boolean groundVisible, Set<Material> hiddenBlocks, Viewers viewers) {
         this.plugin = plugin;
         this.world = world;
         this.region = region;
         this.groundVisible = groundVisible;
         this.hiddenBlocks = hiddenBlocks;
+        this.viewers = viewers;
         this.occluding = new BitSet(region.volume());
         this.visibility = new BitSet(region.volume());
 
@@ -103,33 +103,48 @@ public class Hull {
     }
 
     public boolean isViewer(Player player) {
-        return player != null && viewers.contains(player.getUniqueId());
+        return player != null && viewers.isViewer(player);
     }
 
     /**
-     * Hides the hull from this player (removes the viewer), also hiding all entities currently inside it.
-     */
-    public void removeViewer(Player player) {
-        viewers.remove(player.getUniqueId());
-        for (Entity entity : entities) {
-            if (!player.equals(entity)) untrackEntity(player, entity);
-        }
-    }
-
-    /**
-     * Allows this player to see the hull (adds a viewer).
+     * Adds the player to the {@link Viewers}, which decide whether the player may see the hull contents afterwards.
      */
     public void addViewer(Player player) {
-        if (viewers.add(player.getUniqueId())) {
-            for (Entity entity : entities) {
-                trackEntity(player, entity);
-            }
-            // techhider triggers block change sending
-        }
+        boolean wasViewer = isViewer(player);
+        viewers.addViewer(player);
+        updateEntities(player, wasViewer);
     }
 
+    /**
+     * Removes the player from the {@link Viewers}, which decide whether the player may see the hull contents afterwards.
+     */
+    public void removeViewer(Player player) {
+        boolean wasViewer = isViewer(player);
+        viewers.removeViewer(player);
+        updateEntities(player, wasViewer);
+    }
+
+    private void updateEntities(Player player, boolean wasViewer) {
+        if (isViewer(player)) {
+            if (!wasViewer) {
+                for (Entity entity : entities) {
+                    trackEntity(player, entity);
+                }
+            }
+        } else {
+            // Also hides the entities from players that joined after they were hidden
+            for (Entity entity : entities) {
+                if (!player.equals(entity)) untrackEntity(player, entity);
+            }
+        }
+        // techhider triggers block change sending
+    }
+
+    /**
+     * Resets the player to the default viewer state, e.g. when leaving.
+     */
     public void forgetViewer(Player player) {
-        viewers.remove(player.getUniqueId());
+        viewers.removeViewer(player);
     }
 
     public void checkEntity(Entity entity) {

HiderCore/src/de/steamwar/hullhider/HullHider.java

@@ -107,13 +107,18 @@ public class HullHider<K> implements Listener {
         }
     }
 
-    public void setViewer(Player player, K key, boolean viewer) {
-        Hull hull = hullMap.get(key);
-        if (viewer) {
-            hull.addViewer(player);
-        } else {
-            hull.removeViewer(player);
-        }
+    /**
+     * @see Viewers
+     */
+    public void addViewer(Player player, K key) {
+        hullMap.get(key).addViewer(player);
+    }
+
+    /**
+     * @see Viewers
+     */
+    public void removeViewer(Player player, K key) {
+        hullMap.get(key).removeViewer(player);
     }
 
     public void forgetViewer(Player player) {

HiderCore/src/de/steamwar/hullhider/Viewers.java

@@ -0,0 +1,75 @@
+/*
+ *   This file is a part of the SteamWar software.
+ *
+ *   Copyright (C) 2026 SteamWar.de-Serverteam
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU Affero General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ *   GNU Affero General Public License for more details.
+ *
+ *   You should have received a copy of the GNU Affero General Public License
+ *   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package de.steamwar.hullhider;
+
+import org.bukkit.entity.Player;
+
+import java.util.Set;
+import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
+
+/**
+ * The players of a {@link Hull}. Whether an added player may see the hull contents or sees the hull blocks instead is decided by {@link #isViewer(Player)}.
+ * Stores the players, so every hull needs its own instance.
+ */
+public abstract sealed class Viewers {
+
+    // Concurrent as it is read on netty threads
+    protected final Set<UUID> players = ConcurrentHashMap.newKeySet();
+
+    /**
+     * @return whether the player may see the hull contents instead of the hull blocks
+     */
+    public abstract boolean isViewer(Player player);
+
+    /**
+     * @return whether the player was not added before
+     */
+    public boolean addViewer(Player player) {
+        return players.add(player.getUniqueId());
+    }
+
+    /**
+     * @return whether the player was added before
+     */
+    public boolean removeViewer(Player player) {
+        return players.remove(player.getUniqueId());
+    }
+
+    /**
+     * Hidden from everybody, only added players may see the hull contents.
+     */
+    public static final class HiddenByDefault extends Viewers {
+        @Override
+        public boolean isViewer(Player player) {
+            return players.contains(player.getUniqueId());
+        }
+    }
+
+    /**
+     * Visible to everybody, only added players see the hull blocks instead of the hull contents.
+     */
+    public static final class VisibleByDefault extends Viewers {
+        @Override
+        public boolean isViewer(Player player) {
+            return !players.contains(player.getUniqueId());
+        }
+    }
+}