Do tab list clearing into one packet

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientPlaySessionHandler.java

@@ -240,11 +240,8 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
             player.getConnection().setCanSendLegacyFMLResetPacket(true);
         } else {
             // Clear tab list to avoid duplicate entries
-            TabList tabList = player.getTabList();
-            tabList.getEntries().forEach(entry -> tabList.removeEntry(entry.getProfile().idAsUuid()));
+            player.getTabList().clearAll();
 
-            // Ah, this is the meat and potatoes of the whole venture!
-            //
             // In order to handle switching to another server, you will need to send three packets:
             //
             // - The join game packet from the backend server
@@ -271,7 +268,7 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
         }
         serverBossBars.clear();
 
-        // Tell the server about this client's plugin messages. Velocity will forward them on to the client.
+        // Tell the server about this client's plugin message channels.
         Collection<String> toRegister = new HashSet<>(clientPluginMsgChannels);
         if (player.getProtocolVersion() >= ProtocolConstants.MINECRAFT_1_13) {
             toRegister.addAll(server.getChannelRegistrar().getModernChannelIds());

proxy/src/main/java/com/velocitypowered/proxy/tablist/VelocityTabList.java

@@ -10,12 +10,7 @@ import com.velocitypowered.proxy.protocol.packet.HeaderAndFooter;
 import com.velocitypowered.proxy.protocol.packet.PlayerListItem;
 import net.kyori.text.Component;
 
-import java.util.Collection;
-import java.util.Collections;
-import java.util.HashMap;
-import java.util.Map;
-import java.util.Optional;
-import java.util.UUID;
+import java.util.*;
 import java.util.concurrent.ConcurrentHashMap;
 
 public class VelocityTabList implements TabList {
@@ -60,6 +55,15 @@ public class VelocityTabList implements TabList {
         return Optional.ofNullable(entry);
     }
 
+    public void clearAll() { // Note: this method is called upon server switch
+        List<PlayerListItem.Item> items = new ArrayList<>();
+        for (TabListEntry value : entries.values()) {
+            items.add(PlayerListItem.Item.from(value));
+        }
+        entries.clear();
+        connection.delayedWrite(new PlayerListItem(PlayerListItem.Action.REMOVE_PLAYER, items));
+    }
+
     @Override
     public Collection<TabListEntry> getEntries() {
         return Collections.unmodifiableCollection(this.entries.values());