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;
+    }
+}

CursorLang.Agent/Windows/NativePopupWindow.cs

@@ -0,0 +1,392 @@
+using System.Drawing;
+using System.Runtime.InteropServices;
+using CursorLang.Agent.Interop;
+using CursorLang.Agent.Services;
+using CursorLang.Core.Interop;
+using CursorLang.Core.Models;
+using CursorLang.Core.Services;
+
+namespace CursorLang.Agent.Windows;
+
+/// <summary>
+/// The popup with the short name of the layout, drawn by Win32 alone.
+/// </summary>
+/// <remarks>
+/// A like-for-like replacement of the WPF popup this once was: a rounded rectangle of
+/// radius 4 with 10×4 padding, the fill and the text colour from the settings, the whole
+/// thing at the opacity from the settings, the name in Segoe UI SemiBold at the size
+/// from the settings.
+///
+/// The picture is drawn into an off-screen bitmap and handed to the window whole, by
+/// <c>UpdateLayeredWindow</c>. Painting on demand instead — a <c>WM_PAINT</c> after the
+/// window is shown — is what the first version did, and it had the popup appear holding
+/// the picture of the previous show: hiding a window does not throw its content away,
+/// and the content is always the other layout. Here there is nothing to be stale,
+/// because the window is never shown before its picture is in place.
+///
+/// It also does away with two devices the painted version needed: the corners came from
+/// a window region, which cuts without antialiasing, and the opacity from
+/// <c>SetLayeredWindowAttributes</c>. Both are now just pixels in the bitmap.
+///
+/// Responsible only for showing the popup, its size and its place on screen: when to
+/// take it down is decided by <see cref="LayoutPopupService"/>.
+/// </remarks>
+internal sealed class NativePopupWindow : NativeWindow, ILayoutPopupWindow
+{
+    private const string ClassName = "CursorLang.Agent.Popup";
+
+    // The numbers of the XAML: Border CornerRadius="4" Padding="10,4", all in WPF units
+    private const double CornerRadius = 4;
+    private const double PaddingX = 10;
+    private const double PaddingY = 4;
+
+    private readonly AppSettings _settings;
+
+    private IntPtr _font;
+    private double _fontSize;
+    private double _fontScale;
+
+    internal NativePopupWindow(AppSettings settings)
+        : base(
+            ClassName,
+            "CursorLang popup",
+            WindowNative.WS_POPUP,
+            WindowNative.WS_EX_LAYERED | WindowNative.WS_EX_TOOLWINDOW |
+            WindowNative.WS_EX_NOACTIVATE | WindowNative.WS_EX_TRANSPARENT |
+            WindowNative.WS_EX_TOPMOST)
+    {
+        _settings = settings;
+    }
+
+    /// <summary>
+    /// Shows the popup with the given text at the place set by the settings.
+    /// </summary>
+    public void ShowPopup(string text)
+    {
+        if (Handle == IntPtr.Zero)
+        {
+            return;
+        }
+
+        bool atFixedPoint = _settings.PlacementMode == PopupPlacementMode.FixedPoint;
+        PopupWindowNative.Rect work = default;
+        PopupWindowNative.Rect anchor = default;
+        double scale;
+
+        if (atFixedPoint)
+        {
+            (work, scale) = PopupWindowNative.GetActiveMonitorWorkArea();
+        }
+        else
+        {
+            anchor = GetAnchor();
+            scale = PopupWindowNative.GetScaleAt(new PopupWindowNative.Point { X = anchor.Left, Y = anchor.Top });
+        }
+
+        EnsureFont(scale);
+
+        Size measured = MeasureText(text);
+        int width = measured.Width + (2 * PopupLayout.ToPixels(PaddingX, scale));
+        int height = measured.Height + (2 * PopupLayout.ToPixels(PaddingY, scale));
+
+        PopupWindowNative.Point position = atFixedPoint
+            ? PopupLayout.OnScreen(
+                work, _settings.ScreenPosition, PopupLayout.ToPixels(_settings.ScreenMargin, scale), width, height)
+            : PopupLayout.NearAnchor(anchor, AnchorSideForMode(), OffsetForMode(scale), width, height);
+
+        if (!Draw(text, position, width, height, PopupLayout.ToPixels(CornerRadius, scale)))
+        {
+            return;
+        }
+
+        if (!WindowNative.IsWindowVisible(Handle))
+        {
+            WindowNative.ShowWindow(Handle, WindowNative.SW_SHOWNOACTIVATE);
+        }
+    }
+
+    public void Hide()
+    {
+        if (Handle != IntPtr.Zero)
+        {
+            WindowNative.ShowWindow(Handle, WindowNative.SW_HIDE);
+        }
+    }
+
+    /// <summary>Destroys the window. The agent only does this on the way out.</summary>
+    public void Close() => Dispose();
+
+    public override void Dispose()
+    {
+        ReleaseFont();
+        base.Dispose();
+    }
+
+    /// <summary>
+    /// Draws the popup off screen and hands the finished picture to the window.
+    /// </summary>
+    /// <remarks>
+    /// The bitmap is thrown away afterwards rather than kept: it is a few tens of
+    /// kilobytes for the length of one call, the popup is shown rarely, and a cached one
+    /// would have to be rebuilt on every change of size, colour or scale anyway.
+    /// </remarks>
+    private bool Draw(string text, PopupWindowNative.Point at, int width, int height, int radius)
+    {
+        IntPtr screen = GdiNative.GetDC(IntPtr.Zero);
+        if (screen == IntPtr.Zero)
+        {
+            return false;
+        }
+
+        IntPtr memory = IntPtr.Zero;
+        IntPtr surface = IntPtr.Zero;
+
+        try
+        {
+            memory = GdiNative.CreateCompatibleDC(screen);
+            if (memory == IntPtr.Zero)
+            {
+                return false;
+            }
+
+            surface = GdiNative.CreateSurface(memory, width, height, out IntPtr bits);
+            if (surface == IntPtr.Zero)
+            {
+                return false;
+            }
+
+            GdiNative.SelectObject(memory, surface);
+
+            Fill(bits, width, height);
+            DrawText(memory, text, width, height);
+
+            // GDI writes nothing into the alpha channel, so the letters it just drew are
+            // sitting at zero alpha and would come out invisible. The inside of the
+            // popup is opaque anyway, so the whole surface is simply declared so — and
+            // the corners are rounded off afterwards, which is the only place alpha
+            // varies
+            MakeOpaque(bits, width, height);
+            RoundTheCorners(bits, width, height, radius);
+
+            var size = new WindowNative.Size { Width = width, Height = height };
+            var alpha = (byte)Math.Clamp(Math.Round(_settings.Opacity * 255), 0, 255);
+
+            return WindowNative.SetContent(Handle, at, size, memory, alpha);
+        }
+        finally
+        {
+            // The context goes first: a bitmap still selected into one cannot be
+            // deleted, and this way that holds however the method was left
+            if (memory != IntPtr.Zero)
+            {
+                GdiNative.DeleteDC(memory);
+            }
+
+            if (surface != IntPtr.Zero)
+            {
+                GdiNative.DeleteObject(surface);
+            }
+
+            GdiNative.ReleaseDC(IntPtr.Zero, screen);
+        }
+    }
+
+    private void Fill(IntPtr bits, int width, int height)
+    {
+        Color background = _settings.BackgroundColor;
+
+        // Straight into the bitmap rather than through a brush: the pixels have to be
+        // written anyway to carry an alpha channel GDI would not touch
+        int packed = (255 << 24) | (background.R << 16) | (background.G << 8) | background.B;
+        var row = new int[width];
+        Array.Fill(row, packed);
+
+        for (var y = 0; y < height; y++)
+        {
+            Marshal.Copy(row, 0, bits + (y * width * 4), width);
+        }
+    }
+
+    private void DrawText(IntPtr deviceContext, string text, int width, int height)
+    {
+        if (_font == IntPtr.Zero || text.Length == 0)
+        {
+            return;
+        }
+
+        var bounds = new PopupWindowNative.Rect { Left = 0, Top = 0, Right = width, Bottom = height };
+
+        IntPtr previousFont = GdiNative.SelectObject(deviceContext, _font);
+        GdiNative.SetBkMode(deviceContext, GdiNative.TRANSPARENT);
+        GdiNative.SetTextColor(deviceContext, GdiNative.ToColorRef(_settings.ForegroundColor));
+
+        GdiNative.DrawText(deviceContext, text, text.Length, ref bounds,
+            GdiNative.DT_SINGLELINE | GdiNative.DT_CENTER | GdiNative.DT_VCENTER |
+            GdiNative.DT_NOPREFIX | GdiNative.DT_NOCLIP);
+
+        GdiNative.SelectObject(deviceContext, previousFont);
+    }
+
+    private static void MakeOpaque(IntPtr bits, int width, int height)
+    {
+        var row = new int[width];
+
+        for (var y = 0; y < height; y++)
+        {
+            IntPtr line = bits + (y * width * 4);
+            Marshal.Copy(line, row, 0, width);
+
+            for (var x = 0; x < width; x++)
+            {
+                row[x] = (int)((uint)row[x] | 0xFF000000);
+            }
+
+            Marshal.Copy(row, 0, line, width);
+        }
+    }
+
+    /// <summary>
+    /// Cuts the four corners to a radius, fading the edge rather than stepping it.
+    /// </summary>
+    /// <remarks>
+    /// The painted version cut them with a window region, which is a yes-or-no mask and
+    /// left a visible staircase at 200% scale. Here the corner pixels carry a partial
+    /// alpha worked out from how far the pixel centre is past the arc, which is what
+    /// antialiasing amounts to. The colours are premultiplied to match, as
+    /// <c>UpdateLayeredWindow</c> expects.
+    /// </remarks>
+    private static void RoundTheCorners(IntPtr bits, int width, int height, int radius)
+    {
+        if (radius <= 0)
+        {
+            return;
+        }
+
+        radius = Math.Min(radius, Math.Min(width, height) / 2);
+
+        var row = new int[width];
+
+        for (var y = 0; y < height; y++)
+        {
+            bool nearTop = y < radius;
+            bool nearBottom = y >= height - radius;
+            if (!nearTop && !nearBottom)
+            {
+                continue;
+            }
+
+            IntPtr line = bits + (y * width * 4);
+            Marshal.Copy(line, row, 0, width);
+
+            double centreY = nearTop ? radius - 0.5 : height - radius - 0.5;
+
+            for (var x = 0; x < width; x++)
+            {
+                bool nearLeft = x < radius;
+                bool nearRight = x >= width - radius;
+                if (!nearLeft && !nearRight)
+                {
+                    continue;
+                }
+
+                double centreX = nearLeft ? radius - 0.5 : width - radius - 0.5;
+                double distance = Math.Sqrt(
+                    ((x - centreX) * (x - centreX)) + ((y - centreY) * (y - centreY)));
+
+                // One pixel of softness across the arc: fully inside, fully outside,
+                // and a ramp in between
+                double coverage = Math.Clamp(radius - distance + 0.5, 0, 1);
+                if (coverage >= 1)
+                {
+                    continue;
+                }
+
+                row[x] = Premultiply(row[x], coverage);
+            }
+
+            Marshal.Copy(row, 0, line, width);
+        }
+    }
+
+    private static int Premultiply(int pixel, double coverage)
+    {
+        var value = (uint)pixel;
+        var alpha = (uint)Math.Round(((value >> 24) & 0xFF) * coverage);
+
+        uint red = (uint)Math.Round(((value >> 16) & 0xFF) * coverage);
+        uint green = (uint)Math.Round(((value >> 8) & 0xFF) * coverage);
+        uint blue = (uint)Math.Round((value & 0xFF) * coverage);
+
+        return (int)((alpha << 24) | (red << 16) | (green << 8) | blue);
+    }
+
+    // The anchor point: the caret in the input field or the mouse cursor. The cursor
+    // is a rectangle of zero size, so the corner computation is shared by both
+    private PopupWindowNative.Rect GetAnchor()
+    {
+        if (_settings.PlacementMode == PopupPlacementMode.AtCaret &&
+            CaretNative.TryGetCaretRect() is { } caret)
+        {
+            return caret;
+        }
+
+        return PopupLayout.AsAnchor(PopupWindowNative.GetCursorPosition());
+    }
+
+    // Every anchor mode has a side and an offset of its own
+    private AnchorSide AnchorSideForMode() =>
+        _settings.PlacementMode == PopupPlacementMode.AtCaret ? _settings.CaretSide : _settings.CursorSide;
+
+    private int OffsetForMode(double scale) => PopupLayout.ToPixels(
+        _settings.PlacementMode == PopupPlacementMode.AtCaret ? _settings.CaretOffset : _settings.CursorOffset,
+        scale);
+
+    private Size MeasureText(string text)
+    {
+        IntPtr deviceContext = GdiNative.GetDC(Handle);
+        if (deviceContext == IntPtr.Zero)
+        {
+            return Size.Empty;
+        }
+
+        try
+        {
+            IntPtr previousFont = GdiNative.SelectObject(deviceContext, _font);
+            Size measured = GdiNative.MeasureText(deviceContext, text);
+            GdiNative.SelectObject(deviceContext, previousFont);
+
+            return measured;
+        }
+        finally
+        {
+            GdiNative.ReleaseDC(Handle, deviceContext);
+        }
+    }
+
+    // The font is rebuilt only when the size in the settings or the monitor scale
+    // changes: it is the one expensive thing a show does
+    private void EnsureFont(double scale)
+    {
+        if (_font != IntPtr.Zero &&
+            Math.Abs(_fontSize - _settings.FontSize) < 0.01 &&
+            Math.Abs(_fontScale - scale) < 0.01)
+        {
+            return;
+        }
+
+        ReleaseFont();
+
+        _fontSize = _settings.FontSize;
+        _fontScale = scale;
+        _font = GdiNative.CreateFont(_fontSize, scale);
+    }
+
+    private void ReleaseFont()
+    {
+        if (_font != IntPtr.Zero)
+        {
+            GdiNative.DeleteObject(_font);
+            _font = IntPtr.Zero;
+        }
+    }
+}

CursorLang.Agent/Windows/NativeTrayIcon.cs

@@ -0,0 +1,128 @@
+using CursorLang.Agent.Interop;
+using CursorLang.Core.Interop;
+using CursorLang.Core.Services;
+
+namespace CursorLang.Agent.Windows;
+
+/// <summary>
+/// The icon in the notification area: the way to the settings window and the only way
+/// to quit the application.
+/// </summary>
+/// <remarks>
+/// The interop half is the same as it always was — the icon has never known anything
+/// about the framework. What changed is the menu: a WPF <c>ContextMenu</c> obeyed the
+/// theme and the language chosen in the settings, and a <c>TrackPopupMenuEx</c> menu is
+/// drawn by Windows in the system look. The language still reaches it, because the
+/// captions are ours; the theme does not, and that is the price of taking the rendering
+/// stack out of the background process.
+///
+/// The captions are read each time the menu is raised rather than once: the language is
+/// changed in the settings without a restart, and the menu is built on every click
+/// anyway — a menu costs nothing to build and is asked for rarely.
+/// </remarks>
+internal sealed class NativeTrayIcon : IDisposable
+{
+    /// <summary>Windows shows it under the pointer. The name of the app says enough.</summary>
+    private const string Tooltip = "CursorLang";
+
+    /// <summary>Distinguishes the icon among those of the same window; we have one.</summary>
+    private const int IconId = 1;
+
+    private const int CommandSettings = 1;
+    private const int CommandExit = 2;
+
+    private readonly AgentWindow _window;
+    private readonly ILocalizationService _localization;
+
+    private IntPtr _icon;
+    private bool _isInstalled;
+
+    internal NativeTrayIcon(AgentWindow window, ILocalizationService localization)
+    {
+        _window = window;
+        _localization = localization;
+
+        _window.AddFilter(OnMessage);
+    }
+
+    internal event EventHandler? OpenRequested;
+
+    internal event EventHandler? ExitRequested;
+
+    internal bool Install()
+    {
+        if (_isInstalled)
+        {
+            return true;
+        }
+
+        _icon = TrayIconNative.LoadApplicationIcon();
+        _isInstalled = TrayIconNative.Add(_window.Handle, IconId, _icon, Tooltip);
+
+        return _isInstalled;
+    }
+
+    public void Dispose()
+    {
+        if (_isInstalled)
+        {
+            TrayIconNative.Remove(_window.Handle, IconId);
+            _isInstalled = false;
+        }
+
+        TrayIconNative.ReleaseIcon(_icon);
+        _icon = IntPtr.Zero;
+    }
+
+    private bool OnMessage(uint message, IntPtr wParam, IntPtr lParam)
+    {
+        // Explorer has restarted and taken every icon down with it
+        if (message == (uint)TrayIconNative.TaskbarCreatedMessage && _isInstalled)
+        {
+            TrayIconNative.Add(_window.Handle, IconId, _icon, Tooltip);
+            return true;
+        }
+
+        if (message != TrayIconNative.CallbackMessage)
+        {
+            return false;
+        }
+
+        switch (TrayIconNative.NotificationOf(lParam))
+        {
+            case TrayIconNative.SelectNotification:
+            case TrayIconNative.KeySelectNotification:
+                OpenRequested?.Invoke(this, EventArgs.Empty);
+                return true;
+
+            case TrayIconNative.ContextMenuNotification:
+                ShowMenu(TrayIconNative.PointOf(wParam));
+                return true;
+
+            default:
+                return false;
+        }
+    }
+
+    /// <summary>Raises the menu of the icon where the pointer is.</summary>
+    internal void ShowMenu(PopupWindowNative.Point at)
+    {
+        MenuNative.Item[] items =
+        [
+            new(CommandSettings, _localization["TrayMenuSettings"]),
+            MenuNative.Item.Separator,
+            new(CommandExit, _localization["TrayMenuExit"]),
+        ];
+
+        switch (MenuNative.Track(_window.Handle, at, items))
+        {
+            case CommandSettings:
+                OpenRequested?.Invoke(this, EventArgs.Empty);
+                break;
+
+            case CommandExit:
+                ExitRequested?.Invoke(this, EventArgs.Empty);
+                break;
+        }
+    }
+}

CursorLang.Agent/Windows/NativeWindow.cs

@@ -0,0 +1,104 @@
+using CursorLang.Agent.Interop;
+
+namespace CursorLang.Agent.Windows;
+
+/// <summary>
+/// A window with no framework behind it: a registered class, a handle and a window
+/// procedure that lands in <see cref="OnMessage"/>.
+/// </summary>
+/// <remarks>
+/// Windows knows one procedure per class, so the procedure here is shared and static,
+/// and finds the instance by handle. The very first message of a window arrives while
+/// <c>CreateWindowExW</c> is still running and there is nothing to find yet — that is
+/// what the field holding the instance under construction is for.
+///
+/// Everything is deliberately without locks: the agent has one message loop, every
+/// window belongs to it, and a window procedure can only ever be called on the thread
+/// that created the window.
+/// </remarks>
+internal abstract class NativeWindow : IDisposable
+{
+    private static readonly Dictionary<IntPtr, NativeWindow> Live = [];
+    private static readonly HashSet<string> RegisteredClasses = new(StringComparer.Ordinal);
+
+    // The shared procedure is a static field for the same reason a hook procedure is:
+    // Windows holds the only reference to it and the collector does not see that
+    private static readonly WindowNative.WindowProc SharedProc = StaticWindowProc;
+
+    [ThreadStatic]
+    private static NativeWindow? _creating;
+
+    protected NativeWindow(string className, string title, int style, int exStyle)
+    {
+        if (RegisteredClasses.Add(className))
+        {
+            WindowNative.RegisterClass(className, SharedProc);
+        }
+
+        _creating = this;
+        try
+        {
+            Handle = WindowNative.CreateWindow(className, title, style, exStyle);
+        }
+        finally
+        {
+            _creating = null;
+        }
+
+        Live[Handle] = this;
+    }
+
+    /// <summary>The window handle. Zero once the window is gone.</summary>
+    internal IntPtr Handle { get; private set; }
+
+    public virtual void Dispose()
+    {
+        if (Handle == IntPtr.Zero)
+        {
+            return;
+        }
+
+        IntPtr handle = Handle;
+        Handle = IntPtr.Zero;
+        Live.Remove(handle);
+
+        WindowNative.DestroyWindow(handle);
+    }
+
+    /// <summary>
+    /// A message for this window. Returning <c>false</c> passes it to
+    /// <c>DefWindowProcW</c>, which is what the vast majority of messages want — and
+    /// what all of them want for a window whose content is set from the outside.
+    /// </summary>
+    protected virtual bool OnMessage(uint message, IntPtr wParam, IntPtr lParam, out IntPtr result)
+    {
+        result = IntPtr.Zero;
+        return false;
+    }
+
+    private static IntPtr StaticWindowProc(IntPtr hWnd, uint message, IntPtr wParam, IntPtr lParam)
+    {
+        if (!Live.TryGetValue(hWnd, out NativeWindow? window))
+        {
+            if (_creating is null)
+            {
+                return WindowNative.DefWindowProc(hWnd, message, wParam, lParam);
+            }
+
+            // The window is being created right now: bind the handle to the instance
+            // so that the rest of its creation messages find their way home
+            window = _creating;
+            window.Handle = hWnd;
+            Live[hWnd] = window;
+        }
+
+        if (message == WindowNative.WM_DESTROY)
+        {
+            Live.Remove(hWnd);
+        }
+
+        return window.OnMessage(message, wParam, lParam, out IntPtr result)
+            ? result
+            : WindowNative.DefWindowProc(hWnd, message, wParam, lParam);
+    }
+}