Only write the 1.7 length prefix on Velocity

common/src/main/java/com/viaversion/viarewind/api/ViaRewindPlatform.java

@@ -91,4 +91,13 @@ default String decompressHandlerName() {
         return "decompress";
     }
 
+    /**
+     * Returns whether the platform's compression handler also writes the packet length. Only used in the 1.8 -> 1.7 protocol.
+     *
+     * @return true if the compression handler writes the packet length
+     */
+    default boolean compressHandlerWritesLength() {
+        return false;
+    }
+
 }

common/src/main/java/com/viaversion/viarewind/api/minecraft/netty/ForwardMessageToByteEncoder.java

@@ -24,9 +24,17 @@
 
 public class ForwardMessageToByteEncoder extends MessageToByteEncoder<ByteBuf> {
 
+    private final boolean writeLength;
+
+    public ForwardMessageToByteEncoder(final boolean writeLength) {
+        this.writeLength = writeLength;
+    }
+
     @Override
     protected void encode(ChannelHandlerContext ctx, ByteBuf msg, ByteBuf out) {
-        Types.VAR_INT.writePrimitive(out, msg.readableBytes());
+        if (writeLength) {
+            Types.VAR_INT.writePrimitive(out, msg.readableBytes());
+        }
         out.writeBytes(msg);
     }
 }

common/src/main/java/com/viaversion/viarewind/protocol/v1_8to1_7_6_10/provider/compression/TrackingCompressionHandlerProvider.java

@@ -66,7 +66,7 @@ public void onTransformPacket(UserConnection user) {
 
             if (compressor != null) { // We can neutralize the effect of compressor to the client
                 pipeline.replace(decompressor, decompressor, new EmptyChannelHandler());
-                pipeline.replace(compressor, compressor, new ForwardMessageToByteEncoder());
+                pipeline.replace(compressor, compressor, new ForwardMessageToByteEncoder(ViaRewind.getPlatform().compressHandlerWritesLength()));
             } else {
                 throw new IllegalStateException("Couldn't remove compression for 1.7!");
             }

velocity/src/main/java/com/viaversion/viarewind/VelocityPlugin.java

@@ -79,4 +79,9 @@ public String decompressHandlerName() {
         return "compression-decoder";
     }
 
+    @Override
+    public boolean compressHandlerWritesLength() {
+        return true;
+    }
+
 }