src/main/java/land/chipmunk/chipmunkmod/commands/UsernameCommand.java

@@ -8,63 +8,94 @@ import static com.mojang.brigadier.arguments.StringArgumentType.*;
 import static land.chipmunk.chipmunkmod.command.CommandManager.literal;
 import static land.chipmunk.chipmunkmod.command.CommandManager.argument;
 
+import land.chipmunk.chipmunkmod.mixin.ClientCommonNetworkHandlerAccessor;
+import land.chipmunk.chipmunkmod.util.RandomUtilities;
 import net.fabricmc.fabric.api.client.command.v2.FabricClientCommandSource;
 import net.minecraft.client.MinecraftClient;
+import net.minecraft.client.gui.screen.Screen;
 import net.minecraft.client.gui.screen.TitleScreen;
 import net.minecraft.client.gui.screen.multiplayer.ConnectScreen;
+import net.minecraft.client.network.ClientPlayNetworkHandler;
 import net.minecraft.client.network.ServerInfo;
 import net.minecraft.client.network.ServerAddress;
 import net.minecraft.client.session.Session;
+import net.minecraft.network.ClientConnection;
 import net.minecraft.text.Text;
 import com.mojang.brigadier.exceptions.CommandSyntaxException;
 import com.mojang.brigadier.exceptions.SimpleCommandExceptionType;
 
+import java.util.Objects;
 import java.util.Optional;
-import java.util.UUID;
+import java.util.Random;
 
 import land.chipmunk.chipmunkmod.mixin.MinecraftClientAccessor;
+import net.minecraft.util.Uuids;
 
 public class UsernameCommand {
-    private static final Session ORIGINAL_SESSION = MinecraftClient.getInstance().getSession();
     private static final SimpleCommandExceptionType USERNAME_TOO_LONG = new SimpleCommandExceptionType(Text.translatable("The specified username is longer than 16 characters"));
+    private static final char[] PREMIUM_CHARS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz_"
+            .toCharArray();
+    private static final char SECTION_CHAR = '§';
+
+    private static final Session ORIGINAL_SESSION = MinecraftClient.getInstance().getSession();
+    private static final Random RANDOM = new Random();
 
     public static void register(CommandDispatcher<FabricClientCommandSource> dispatcher) {
-        dispatcher.register(
-                literal("username")
-                        .then(
-                                literal("set")
-                                        .then(
-                                                argument("username", string())
-                                                        .executes(UsernameCommand::updateUsername)
-                                        )
-                        )
-                        .then(
-                                literal("revert")
-                                        .executes(c -> updateSession(c, ORIGINAL_SESSION))
-                        )
+        dispatcher.register(literal("username")
+                .then(literal("set")
+                        .then(argument("username", string())
+                                .executes(c -> changeOffline(c, getString(c, "username")))))
+                .then(literal("revert")
+                        .executes(c -> changeSession(c, ORIGINAL_SESSION)))
+
+                .then(literal("random")
+                        .then(literal("blank")
+                                .executes(c -> changeOffline(c, RandomUtilities.emptyUsername(RANDOM, SECTION_CHAR))))
+                        .executes(c -> changeOffline(c, RandomUtilities.randomString(RANDOM, PREMIUM_CHARS, RANDOM.nextInt(3, 16)))))
+                .then(literal("empty")
+                        .executes(c -> changeOffline(c, "")))
         );
     }
 
-    public static int updateUsername(CommandContext<FabricClientCommandSource> context) throws CommandSyntaxException {
-        final String username = getString(context, "username");
-        if (username.length() > 16) throw USERNAME_TOO_LONG.create();
-        final Session session = new Session(username, new UUID(0L, 0L), "", Optional.empty(), Optional.empty(), Session.AccountType.MOJANG);
-        return updateSession(context, session);
+    private static Session offline(final String username) {
+        // This is how Minecraft's Main class does it
+        return new Session(username, Uuids.getOfflinePlayerUuid(username),
+                "", Optional.empty(), Optional.empty(), Session.AccountType.LEGACY);
     }
 
-    public static int updateSession(CommandContext<FabricClientCommandSource> context, Session session) {
-        final FabricClientCommandSource source = context.getSource();
+    // TODO: Put this in a separate class
+    private static void reconnect(final MinecraftClient client) {
+        final ClientPlayNetworkHandler networkHandler = client.getNetworkHandler();
+        if (networkHandler == null) return; // single-player?
 
-        final MinecraftClient client = source.getClient();
+        final ServerInfo info = networkHandler.getServerInfo();
+        if (info == null) return; // definitely single-player
 
+        final ClientConnection connection = networkHandler.getConnection();
+        final Screen screen = Objects.requireNonNullElseGet(
+                ((ClientCommonNetworkHandlerAccessor) networkHandler).getPostDisconnectScreen(),
+                TitleScreen::new);
+
+        // This stuff needs to run after we close chat, otherwise it kicks us to the title screen. Don't ask me why
+        client.send(() -> {
+            connection.disconnect(Text.translatable("disconnect.transfer"));
+            connection.tryDisableAutoRead();
+            connection.handleDisconnection();
+
+            ConnectScreen.connect(screen, client, ServerAddress.parse(info.address), info, false, null);
+        });
+    }
+
+    private static int changeSession(final CommandContext<FabricClientCommandSource> context, final Session session) {
+        final MinecraftClient client = context.getSource().getClient();
         ((MinecraftClientAccessor) client).session(session);
 
-        // TODO: Put this in a separate class
-        final ServerInfo info = client.getCurrentServerEntry();
-        if (client.world != null) client.world.disconnect();
-        client.disconnect();
-        ConnectScreen.connect(new TitleScreen(), client, ServerAddress.parse(info.address), info, false, null);
-
+        reconnect(client);
         return Command.SINGLE_SUCCESS;
     }
+
+    private static int changeOffline(final CommandContext<FabricClientCommandSource> context, final String username) throws CommandSyntaxException {
+        if (username.length() > 16) throw USERNAME_TOO_LONG.create();
+        return changeSession(context, offline(username));
+    }
 }

src/main/java/land/chipmunk/chipmunkmod/mixin/ClientCommonNetworkHandlerAccessor.java

@@ -0,0 +1,12 @@
+package land.chipmunk.chipmunkmod.mixin;
+
+import net.minecraft.client.gui.screen.Screen;
+import net.minecraft.client.network.ClientCommonNetworkHandler;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.gen.Accessor;
+
+@Mixin(ClientCommonNetworkHandler.class)
+public interface ClientCommonNetworkHandlerAccessor {
+    @Accessor("postDisconnectScreen")
+    Screen getPostDisconnectScreen();
+}

src/main/java/land/chipmunk/chipmunkmod/util/RandomUtilities.java

@@ -9,12 +9,16 @@ public final class RandomUtilities {
             .toCharArray();
 
     public static String emptyUsername(final Random random) {
+        return RandomUtilities.emptyUsername(random, '&');
+    }
+
+    public static String emptyUsername(final Random random, char colorChar) {
         final char[] buf = new char[16];
 
         for (int i = 0; i < 16; i += 2) {
             final int j = random.nextInt(LEGACY_STYLE_CODES.length);
 
-            buf[i] = '&';
+            buf[i] = colorChar;
             buf[i + 1] = LEGACY_STYLE_CODES[j];
         }
 

src/main/resources/chipmunkmod.mixins.json

@@ -6,6 +6,7 @@
   "client": [
     "ChatInputSuggestorMixin",
     "ChatScreenMixin",
+    "ClientCommonNetworkHandlerAccessor",
     "ClientConnectionMixin",
     "ClientPlayerEntityMixin",
     "ClientPlayNetworkHandlerMixin",