Got at least one data fetching method working; turns out, we can't use a patched LogicStack to get the data

Vendor/EmbedIO-3.5.2/WebSockets/Internal/MessageEventArgs.cs

@@ -0,0 +1,114 @@
+using System;
+using Swan;
+
+namespace EmbedIO.WebSockets.Internal
+{
+    /// <summary>
+    /// Represents the event data for the <see cref="Internal.WebSocket.OnMessage"/> event.
+    /// </summary>
+    /// <remarks>
+    ///   <para>
+    ///   That event occurs when the <see cref="Internal.WebSocket"/> receives
+    ///   a message or a ping if the <see cref="Internal.WebSocket.EmitOnPing"/>
+    ///   property is set to <c>true</c>.
+    ///   </para>
+    ///   <para>
+    ///   If you would like to get the message data, you should access
+    ///   the <see cref="Data"/> or <see cref="RawData"/> property.
+    ///   </para>
+    /// </remarks>
+    internal class MessageEventArgs : EventArgs
+    {
+        private readonly byte[] _rawData;
+        private string? _data;
+        private bool _dataSet;
+
+        internal MessageEventArgs(WebSocketFrame frame)
+        {
+            Opcode = frame.Opcode;
+            _rawData = frame.PayloadData.ApplicationData.ToArray();
+        }
+
+        internal MessageEventArgs(Opcode opcode, byte[] rawData)
+        {
+            if ((ulong)rawData.Length > PayloadData.MaxLength)
+                throw new WebSocketException(CloseStatusCode.TooBig);
+
+            Opcode = opcode;
+            _rawData = rawData;
+        }
+
+        /// <summary>
+        /// Gets the message data as a <see cref="string"/>.
+        /// </summary>
+        /// <value>
+        /// A <see cref="string"/> that represents the message data if its type is
+        /// text or ping and if decoding it to a string has successfully done;
+        /// otherwise, <see langword="null"/>.
+        /// </value>
+        public string? Data
+        {
+            get
+            {
+                SetData();
+                return _data;
+            }
+        }
+
+        /// <summary>
+        /// Gets a value indicating whether the message type is binary.
+        /// </summary>
+        /// <value>
+        /// <c>true</c> if the message type is binary; otherwise, <c>false</c>.
+        /// </value>
+        public bool IsBinary => Opcode == Opcode.Binary;
+
+        /// <summary>
+        /// Gets a value indicating whether the message type is ping.
+        /// </summary>
+        /// <value>
+        /// <c>true</c> if the message type is ping; otherwise, <c>false</c>.
+        /// </value>
+        public bool IsPing => Opcode == Opcode.Ping;
+
+        /// <summary>
+        /// Gets a value indicating whether the message type is text.
+        /// </summary>
+        /// <value>
+        /// <c>true</c> if the message type is text; otherwise, <c>false</c>.
+        /// </value>
+        public bool IsText => Opcode == Opcode.Text;
+
+        /// <summary>
+        /// Gets the message data as an array of <see cref="byte"/>.
+        /// </summary>
+        /// <value>
+        /// An array of <see cref="byte"/> that represents the message data.
+        /// </value>
+        public byte[] RawData
+        {
+            get
+            {
+                SetData();
+                return _rawData;
+            }
+        }
+
+        internal Opcode Opcode { get; }
+
+        private void SetData()
+        {
+            if (_dataSet)
+                return;
+
+            if (Opcode == Opcode.Binary)
+            {
+                _dataSet = true;
+                return;
+            }
+
+            _data = _rawData.ToText();
+            _dataSet = true;
+        }
+    }
+}