cursor-lang/CursorLang.Settings/Services/MainWindowPlacement.cs

using System.Windows;
using System.Windows.Interop;
using System.Windows.Media;
using System.Windows.Threading;
using CursorLang.Core.Interop;
using CursorLang.Settings.Interop;

namespace CursorLang.Settings.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;

    // Captured once, before anything narrows MaxHeight to a particular monitor —
    // otherwise a later, more generous monitor would stay capped at whatever a
    // previous, shorter one left behind
    private double? _designMaxHeight;

    // Captured once, so a DPI change has a known-correct value to reassert — see
    // RestoreDesignWidth
    private double? _designWidth;

    /// <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;
        window.DpiChanged += OnDpiChanged;
    }

    /// <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;
        _designMaxHeight = window.MaxHeight;
        _designWidth = window.Width;

        LimitHeightToOwnMonitor(window);
        window.UpdateLayout();
        Apply(window);
    }

    // A monitor is not necessarily final at creation time — Windows may place the new
    // window on one monitor before Apply moves it to another, and the user is free to
    // drag it to a third one later. Every one of those is a real DPI change, and this
    // runs again for each: recomputing the cap from whichever monitor holds the window
    // right now, rather than trusting a value worked out for a previous one.
    private void OnDpiChanged(object sender, DpiChangedEventArgs e)
    {
        if (sender is not Window window)
        {
            return;
        }

        // Deferred rather than run inline: this event fires while WPF is still in
        // the middle of its own response to the same DPI change (its per-monitor
        // rescale of Width touches it after this handler if the fix-up runs
        // synchronously, undoing it). Posting behind that on the dispatcher queue
        // lets our fix-up run once WPF's own pass has finished.
        window.Dispatcher.BeginInvoke(DispatcherPriority.ContextIdle, new Action(() =>
        {
            LimitHeightToOwnMonitor(window);
            RestoreDesignWidth(window);
            ReapplySizeToContent(window);
        }));
    }

    // The width is a plain, explicit value rather than something SizeToContent
    // computes, and WPF's own per-monitor rescaling does not reliably keep it at the
    // same logical width when the system's scaling changes live under an
    // already-open window, as opposed to the window being dragged onto a different
    // monitor — it can come out scaled by roughly the ratio between the old and the
    // new DPI instead of staying put. Reasserting the original value here is simpler
    // than chasing exactly where that rescale goes wrong.
    private void RestoreDesignWidth(Window window)
    {
        if (_designWidth is { } designWidth)
        {
            window.Width = designWidth;
        }
    }

    // UpdateLayout alone settles the measure/arrange pass of the visual tree, but
    // does not reliably make WPF redo its own step of resizing the native window to
    // match SizeToContent when the change originates from a DPI event rather than an
    // ordinary content change. Left alone, the window can end up either too tall — a
    // blank strip below the real content, once MaxHeight has just pulled the content
    // shorter — or stuck too short after being dragged back to a monitor with room to
    // grow again. Turning SizeToContent off and back on forces that resizing step to
    // run again from scratch.
    private static void ReapplySizeToContent(Window window)
    {
        SizeToContent original = window.SizeToContent;
        window.SizeToContent = SizeToContent.Manual;
        window.SizeToContent = original;
        window.UpdateLayout();
    }

    // MaxHeight in XAML is a constant tuned for an ordinary desktop monitor at 100%
    // scaling. At a high scale factor the same number of device-independent pixels
    // turns into more physical pixels than a short or heavily scaled monitor has, and
    // SizeToContent then grows the window past the bottom of the screen instead of
    // asking the ScrollViewer inside it to scroll — with nothing to grab, the excess
    // is unreachable. Capping MaxHeight to what the window's own monitor really offers
    // keeps the whole window on screen and lets the ScrollViewer take over.
    private void LimitHeightToOwnMonitor(Window window)
    {
        if (_designMaxHeight is not { } designMaxHeight)
        {
            return;
        }

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

        double scale = VisualTreeHelper.GetDpi(window).DpiScaleY;
        if (scale <= 0)
        {
            return;
        }

        // Leaves room for the title bar and a margin from the edges of the screen,
        // in the same units as the work area height once the monitor's scale is
        // divided out
        const double reservedForChrome = 80;

        double workAreaHeight = (work.Bottom - work.Top) / scale;
        window.MaxHeight = Math.Min(designMaxHeight, Math.Max(200, workAreaHeight - reservedForChrome));
    }

    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();
        return IsEmpty(work) ? null : Center(bounds, work);
    }

    /// <summary>
    /// The point at which a window with the given bounds ends up in the centre of the work area.
    /// </summary>
    internal static PopupWindowNative.Point Center(
        PopupWindowNative.Rect bounds, PopupWindowNative.Rect work) => new()
        {
            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 Clamp(position, bounds, work);
    }

    /// <summary>
    /// Pulls the point so that a window with the given bounds fits into the work area
    /// entirely. A window taller than the work area gets its top edge: the title bar
    /// is needed more than the lower part of the window.
    /// </summary>
    internal static PopupWindowNative.Point Clamp(
        PopupWindowNative.Point position,
        PopupWindowNative.Rect bounds,
        PopupWindowNative.Rect work)
    {
        int width = Width(bounds);
        int height = Height(bounds);

        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;

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