lightweight variant (#1)

Reviewed-on: #1
Co-authored-by: Aleksandr Neychev <alexnejchev73@gmail.com>

CursorLang.Agent/Windows/AgentWindow.cs

@@ -0,0 +1,85 @@
+using System.Collections.Concurrent;
+using CursorLang.Agent.Interop;
+
+namespace CursorLang.Agent.Windows;
+
+/// <summary>
+/// The window the agent lives around: never shown, but it owns the tray icon and it
+/// is the way back onto the message loop from a callback.
+/// </summary>
+/// <remarks>
+/// A window with no <c>WS_VISIBLE</c> shows nowhere, yet is a window in every other
+/// way. A message-only window would do as well were it not for the news of Explorer
+/// restarting: that one is broadcast, and broadcasts pass such windows by.
+/// </remarks>
+internal sealed class AgentWindow : NativeWindow
+{
+    /// <summary>
+    /// A message hook. Returning <c>true</c> means the message has been dealt with.
+    /// </summary>
+    internal delegate bool MessageFilter(uint message, IntPtr wParam, IntPtr lParam);
+
+    private const string ClassName = "CursorLang.Agent.Window";
+
+    /// <summary>Drain the queue of posted work. WM_APP is free for the application.</summary>
+    private const uint WM_INVOKE = WindowNative.WM_APP + 100;
+
+    private readonly List<MessageFilter> _filters = [];
+    private readonly ConcurrentQueue<Action> _posted = new();
+
+    internal AgentWindow()
+        : base(ClassName, "CursorLang agent", WindowNative.WS_POPUP, WindowNative.WS_EX_TOOLWINDOW)
+    {
+    }
+
+    internal void AddFilter(MessageFilter filter) => _filters.Add(filter);
+
+    /// <summary>
+    /// Runs the action on the message loop, after the current message is done with.
+    /// </summary>
+    /// <remarks>
+    /// This is what the agent has instead of <c>Dispatcher.BeginInvoke</c>. The caller
+    /// that matters is the keyboard hook: Windows removes a hook whose procedure takes
+    /// too long, so the procedure only records what happened and the answer — showing
+    /// the popup, switching the layout — waits for the message after this one.
+    /// </remarks>
+    internal void Post(Action action)
+    {
+        _posted.Enqueue(action);
+        WindowNative.PostMessage(Handle, WM_INVOKE, IntPtr.Zero, IntPtr.Zero);
+    }
+
+    /// <summary>Asks the message loop to finish.</summary>
+    internal void Quit() => WindowNative.PostQuitMessage(0);
+
+    protected override bool OnMessage(uint message, IntPtr wParam, IntPtr lParam, out IntPtr result)
+    {
+        result = IntPtr.Zero;
+
+        if (message == WM_INVOKE)
+        {
+            while (_posted.TryDequeue(out Action? action))
+            {
+                action();
+            }
+
+            return true;
+        }
+
+        if (message is WindowNative.WM_CLOSE or WindowNative.WM_ENDSESSION)
+        {
+            Quit();
+            return true;
+        }
+
+        foreach (MessageFilter filter in _filters)
+        {
+            if (filter(message, wParam, lParam))
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
+}