Implement #149 to send just one respawn packet

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

@@ -304,25 +304,19 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
       // Clear tab list to avoid duplicate entries
       player.getTabList().clearAll();
 
-      // In order to handle switching to another server, you will need to send three packets:
+      // In order to handle switching to another server, you will need to send two packets:
       //
-      // - The join game packet from the backend server
-      // - A respawn packet with a different dimension
-      // - Another respawn with the correct dimension
-      //
-      // The two respawns with different dimensions are required, otherwise the client gets
-      // confused.
+      // - The join game packet from the backend server, with a different dimension
+      // - A respawn with the correct dimension
       //
       // Most notably, by having the client accept the join game packet, we can work around the need
       // to perform entity ID rewrites, eliminating potential issues from rewriting packets and
       // improving compatibility with mods.
+      int realDim = joinGame.getDimension();
+      joinGame.setDimension(getFakeTemporaryDimensionId(realDim));
       player.getConnection().delayedWrite(joinGame);
-      int tempDim = joinGame.getDimension() == 0 ? -1 : 0;
       player.getConnection().delayedWrite(
-          new Respawn(tempDim, joinGame.getDifficulty(), joinGame.getGamemode(),
-              joinGame.getLevelType()));
-      player.getConnection().delayedWrite(
-          new Respawn(joinGame.getDimension(), joinGame.getDifficulty(), joinGame.getGamemode(),
+          new Respawn(realDim, joinGame.getDifficulty(), joinGame.getGamemode(),
               joinGame.getLevelType()));
     }
 
@@ -360,11 +354,14 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
     destination.completeJoin();
   }
 
+  private static int getFakeTemporaryDimensionId(int dim) {
+    return dim == 0 ? -1 : 0;
+  }
+
   public List<UUID> getServerBossBars() {
     return serverBossBars;
   }
 
-
   private boolean handleCommandTabComplete(TabCompleteRequest packet) {
     // In 1.13+, we need to do additional work for the richer suggestions available.
     String command = packet.getCommand().substring(1);