cursor-lang/CursorLang/Services/MainWindowPlacement.cs

using System.Windows;
using System.Windows.Interop;
using CursorLang.Interop;

namespace CursorLang.Services;

/// <summary>
/// Decides where the settings window shows up: for the first time in a session — in
/// the centre of the monitor the user is working on, and after that — where they
/// left that window.
/// </summary>
/// <remarks>
/// The position lives in memory only and is not kept between launches: the set of
/// monitors may be different by the next launch, while "in the centre of the active
/// one" is always right.
/// </remarks>
public sealed class MainWindowPlacement
{
    private PopupWindowNative.Point? _position;

    // The window reports a move when we move it ourselves as well;
    // what has to be remembered is only what the user chose
    private bool _isPlacing;

    /// <summary>
    /// Takes over the placement of the window: puts it in place by the first show
    /// and follows where the user moves it.
    /// </summary>
    public void Attach(Window window)
    {
        window.SourceInitialized += OnSourceInitialized;
        window.LocationChanged += OnLocationChanged;
    }

    /// <summary>
    /// Returns the window to the remembered place, and when it has not been shown
    /// yet during this session — puts it in the centre of the active monitor.
    /// </summary>
    public void Apply(Window window)
    {
        // A minimized window has no meaningful bounds: it is restored in its former
        // place, and it can be positioned only after that
        if (window.WindowState != WindowState.Normal)
        {
            return;
        }

        IntPtr handle = new WindowInteropHelper(window).Handle;
        if (handle == IntPtr.Zero || WindowPlacementNative.TryGetBounds(handle) is not { } bounds)
        {
            return;
        }

        PopupWindowNative.Point? wanted = _position ?? CenterOnActiveMonitor(bounds);
        if (wanted is null || KeepOnScreen(wanted.Value, bounds) is not { } target)
        {
            return;
        }

        _isPlacing = true;
        try
        {
            PopupWindowNative.MoveTo(handle, target.X, target.Y);
        }
        finally
        {
            _isPlacing = false;
        }

        _position = target;
    }

    // The window height adapts to its content and is unknown until the first layout
    // pass — an empty window frame would end up in the centre. So we ask for the
    // layout to be computed right away: by that moment the window is not shown yet,
    // so it will not flash in its former place
    private void OnSourceInitialized(object? sender, EventArgs e)
    {
        if (sender is not Window window)
        {
            return;
        }

        window.SourceInitialized -= OnSourceInitialized;
        window.UpdateLayout();
        Apply(window);
    }

    private void OnLocationChanged(object? sender, EventArgs e)
    {
        if (_isPlacing || sender is not Window { WindowState: WindowState.Normal } window)
        {
            return;
        }

        IntPtr handle = new WindowInteropHelper(window).Handle;
        if (handle != IntPtr.Zero && WindowPlacementNative.TryGetBounds(handle) is { } bounds)
        {
            _position = new PopupWindowNative.Point { X = bounds.Left, Y = bounds.Top };
        }
    }

    private static PopupWindowNative.Point? CenterOnActiveMonitor(PopupWindowNative.Rect bounds)
    {
        (PopupWindowNative.Rect work, _) = PopupWindowNative.GetActiveMonitorWorkArea();
        if (IsEmpty(work))
        {
            return null;
        }

        return new PopupWindowNative.Point
        {
            X = work.Left + (((work.Right - work.Left) - Width(bounds)) / 2),
            Y = work.Top + (((work.Bottom - work.Top) - Height(bounds)) / 2),
        };
    }

    /// <summary>
    /// Pulls the window into the work area of the nearest monitor.
    /// </summary>
    /// <remarks>
    /// Needed in two cases. The monitor the user put the window on may be disconnected
    /// during the session — returning the window to its place would then leave the
    /// user without a window, so the remembered position is a wish here rather than an
    /// order. And the window height equals the height of its content and may exceed
    /// the work area on a short monitor — then the title bar of a window placed in the
    /// centre would go past the top edge.
    /// </remarks>
    private static PopupWindowNative.Point? KeepOnScreen(
        PopupWindowNative.Point position, PopupWindowNative.Rect bounds)
    {
        int width = Width(bounds);
        int height = Height(bounds);

        var wanted = new PopupWindowNative.Rect
        {
            Left = position.X,
            Top = position.Y,
            Right = position.X + width,
            Bottom = position.Y + height,
        };

        if (WindowPlacementNative.TryGetWorkAreaNear(wanted) is not { } work || IsEmpty(work))
        {
            return null;
        }

        return new PopupWindowNative.Point
        {
            X = Math.Clamp(position.X, work.Left, Math.Max(work.Left, work.Right - width)),
            Y = Math.Clamp(position.Y, work.Top, Math.Max(work.Top, work.Bottom - height)),
        };
    }

    private static int Width(PopupWindowNative.Rect rect) => rect.Right - rect.Left;

    private static int Height(PopupWindowNative.Rect rect) => rect.Bottom - rect.Top;

    private static bool IsEmpty(PopupWindowNative.Rect rect) =>
        rect.Right <= rect.Left || rect.Bottom <= rect.Top;
}