acdream/src/AcDream.App/UI/UiElement.cs

using System;
using System.Collections.Generic;
using System.Numerics;

namespace AcDream.App.UI;

/// <summary>Which parent edges a child keeps a fixed margin to on resize.
/// Left+Right ⇒ width stretches; Top+Bottom ⇒ height stretches.</summary>
[System.Flags]
public enum AnchorEdges { None = 0, Left = 1, Top = 2, Right = 4, Bottom = 8 }

/// <summary>Retail dat cursor media attached to a UI state.</summary>
public readonly record struct UiCursorMedia(uint File, int HotspotX, int HotspotY)
{
    public bool IsValid => File != 0;
}

/// <summary>
/// Base class for every UI widget in the retained-mode tree.
///
/// Design notes:
/// - Retail AC delegates widget semantics to the external
///   <c>keystone.dll</c> library (see
///   <c>docs/research/retail-ui/02-class-hierarchy.md</c> — there is no
///   widget hierarchy inside <c>acclient.exe</c> itself). We implement
///   our own retained-mode toolkit here, matching the <i>behavior</i>
///   described in the decompile without trying to byte-match Keystone's
///   internal class layout.
/// - Events use the retail-faithful <see cref="UiEvent"/> struct and
///   the <see cref="UiEventType"/> constants so that hand-ported panel
///   code can use the same magic numbers the decompiled C uses
///   (e.g. <c>if (e.Type == 0x15) ...</c> for drag-begin).
/// - Hit-testing is children-first (topmost wins) with Z-order tie
///   breaking; drawing is back-to-front so later children appear on top.
/// - Coordinates are in <b>screen pixels</b>, origin top-left.
///   <see cref="Bounds"/> is in the parent's local coordinate space.
/// </summary>
public abstract class UiElement
{
    // ── Identity ─────────────────────────────────────────────────────────
    /// <summary>
    /// Unique 32-bit event ID. Retail uses the range <c>0x10000000+</c>
    /// for custom app events (see
    /// <c>docs/research/retail-ui/04-input-events.md §3</c>). Assigned
    /// by <see cref="UiRoot"/> when the element is added to the tree.
    /// </summary>
    public uint EventId { get; internal set; }

    /// <summary>
    /// Dat LayoutDesc element id for widgets imported from retail UI data.
    /// Zero for runtime-created helper widgets. This is intentionally separate
    /// from <see cref="EventId"/>: event ids are runtime-local, while this id is
    /// stable across retail layout dumps, decomp references, and UI probes.
    /// </summary>
    public uint DatElementId { get; internal set; }

    /// <summary>Human-readable name for debugging / FindByName.</summary>
    public string? Name { get; init; }

    /// <summary>
    /// GF-13 (Campaign CC gate round 1, Batch A): mirrors
    /// <c>ElementInfo.Invisible</c> (dat property <c>0x3B</c>) — a PURE DATA
    /// PASSTHROUGH set by <c>LayoutImporter.BuildWidget</c> at construction.
    /// The shared importer does NOT act on this flag (1,083 elements author
    /// it client-wide, docs/ISSUES.md #408); it exists only so a screen that
    /// owns its own mounted subtree can honor it explicitly, the way
    /// <c>CharacterCreationUiController</c> does for the chargen screen
    /// (register AP-230). Reading this never changes <see cref="Visible"/> by
    /// itself.
    /// </summary>
    public bool AuthoredInvisible { get; internal set; }

    /// <summary>
    /// #409 (client-wide retail tooltip system): mirrors
    /// <c>ElementInfo.TooltipEnabled</c> (dat property <c>0x4B</c>) — a
    /// PURE DATA PASSTHROUGH set by <c>LayoutImporter.BuildWidget</c>.
    /// Gates whether <see cref="RetailTooltipPresenter"/> may show a
    /// tooltip for this widget at all (retail's per-element
    /// <c>UIRegion::SetTooltipOn</c> bit, checked by
    /// <c>UIElement::MouseHover @0x00462520</c> alongside the global
    /// enable preference). See <see cref="AcDream.App.UI.Layout.ElementInfo.TooltipEnabled"/>.
    /// </summary>
    public bool AuthoredTooltipEnabled { get; internal set; }

    /// <summary>
    /// #409: mirrors <c>ElementInfo.TooltipText</c> (dat property
    /// <c>0x49</c>), already resolved to a display string through
    /// <c>DatStringResolver</c> and escape-normalized at import time —
    /// the same treatment every other authored <c>StringInfo</c> caption
    /// gets. Null when the element authors no tooltip text.
    /// </summary>
    public string? AuthoredTooltipText { get; internal set; }

    /// <summary>
    /// #409: mirrors <c>ElementInfo.TooltipRootElementId</c> (dat property
    /// <c>0x47</c>) — the element-desc id WITHIN
    /// <see cref="AuthoredTooltipLayoutDid"/>'s LayoutDesc to instantiate
    /// as the tooltip popup's root. Zero when absent.
    /// </summary>
    public uint AuthoredTooltipRootElementId { get; internal set; }

    /// <summary>
    /// #409: mirrors <c>ElementInfo.TooltipLayoutDid</c> (dat property
    /// <c>0x48</c>) — the tooltip popup LayoutDesc DID (retail authors
    /// <c>0x21000041</c> on every tooltip-bearing element). Zero when
    /// absent.
    /// </summary>
    public uint AuthoredTooltipLayoutDid { get; internal set; }

    /// <summary>
    /// #409 (live-failure round): the DID of the <c>LayoutDesc</c> this widget
    /// was imported from, stamped by <c>LayoutImporter.Import</c>'s dat shell.
    /// Retail's <c>UIElement::StartTooltipAtMouse @0x00460D70</c> falls back to
    /// exactly this — <c>this->m_layout->m_DID</c> (<c>@0x00460E7E</c>) — when
    /// the hovered element authors a tooltip popup ROOT (<c>P0x47</c>) but no
    /// popup LAYOUT (<c>P0x48</c>), i.e. the popup root lives in the element's
    /// own layout. Zero when the widget came through the pure
    /// <c>LayoutImporter.Build</c> layer (which has no dat context) or was
    /// constructed directly; the presenter then behaves exactly as it did
    /// before this field existed.
    /// </summary>
    public uint SourceLayoutDid { get; internal set; }

    /// <summary>
    /// Optional game object represented by this retained widget for retail's
    /// global found-object cursor. Most widgets leave this unset. The toolbar
    /// inventory/backpack button supplies the player object: retail treats that
    /// button as the self/backpack target (the same target used by its click and
    /// drop paths), so hovering it selects the Found cursor family even though
    /// the authored element is a Button rather than a UIItem.
    /// </summary>
    public Func<uint>? FoundObjectGuidProvider { get; set; }

    /// <summary>
    /// #409: mirrors <c>ElementInfo.TooltipTextChildElementId</c> (dat
    /// property <c>0x4A</c>). Meaningful only when read off a tooltip
    /// POPUP's own instantiated ROOT widget — see the
    /// <c>ElementInfo</c> field's own doc for why retail reads this off
    /// the popup, not the hovering trigger element.
    /// </summary>
    public uint AuthoredTooltipTextChildElementId { get; internal set; }

    /// <summary>
    /// #409: mirrors <c>ElementInfo.TooltipDelaySeconds</c> (dat property
    /// <c>0x50</c>) — a per-element hover-dwell override in seconds, used
    /// by <see cref="UiRoot"/>'s tooltip timer in place of the global
    /// <see cref="UiRoot.TooltipDelayMs"/> when present. Null = no override.
    /// </summary>
    public float? AuthoredTooltipDelaySeconds { get; internal set; }

    /// <summary>
    /// #409 F8: mirrors <c>ElementInfo.MaxWidth</c>/<c>MinWidth</c> (dat
    /// properties <c>0x3D</c>/<c>0x3F</c>) — the <c>UIElement::ResizeTo
    /// @0x00463C30</c> auto-resize width clamp. Null = no authored
    /// override on that side.
    /// </summary>
    public int? AuthoredResizeMaxWidth { get; internal set; }
    public int? AuthoredResizeMinWidth { get; internal set; }

    /// <summary>
    /// #409 F8: mirrors <c>ElementInfo.MaxHeight</c>/<c>MinHeight</c> (dat
    /// properties <c>0x3C</c>/<c>0x3E</c>) — the height side of the same
    /// <c>UIElement::ResizeTo @0x00463C30</c> clamp. Null = no authored
    /// override on that side.
    /// </summary>
    public int? AuthoredResizeMaxHeight { get; internal set; }
    public int? AuthoredResizeMinHeight { get; internal set; }

    private readonly Dictionary<string, UiCursorMedia> _stateCursors = new();

    /// <summary>Retail MediaDescCursor entries keyed by UIStateId.ToString(), or "" for DirectState.</summary>
    public IReadOnlyDictionary<string, UiCursorMedia> StateCursors => _stateCursors;

    /// <summary>Active state name used for cursor media. Stateful dat widgets override this.</summary>
    public virtual string ActiveCursorStateName => "";

    public void SetStateCursors(IReadOnlyDictionary<string, UiCursorMedia> cursors)
    {
        _stateCursors.Clear();
        foreach (var kv in cursors)
        {
            if (kv.Value.IsValid)
                _stateCursors[kv.Key] = kv.Value;
        }
    }

    public UiCursorMedia ActiveCursor()
        => CursorForState(ActiveCursorStateName, allowFallback: true);

    public UiCursorMedia CursorForState(string stateName, bool allowFallback = true)
    {
        if (!string.IsNullOrEmpty(stateName)
            && _stateCursors.TryGetValue(stateName, out var named)
            && named.IsValid)
            return named;

        if (!allowFallback)
            return default;

        if (_stateCursors.TryGetValue("", out var direct) && direct.IsValid)
            return direct;
        if (_stateCursors.TryGetValue("Normal", out var normal) && normal.IsValid)
            return normal;

        return default;
    }

    // ── Geometry ────────────────────────────────────────────────────────
    /// <summary>X in the parent's local pixel space.</summary>
    public float Left   { get; set; }
    public float Top    { get; set; }
    public float Width  { get; set; }
    public float Height { get; set; }

    /// <summary>Absolute (screen-space) top-left, computed by walking Parent.</summary>
    public Vector2 ScreenPosition
    {
        get
        {
            var p = new Vector2(Left, Top);
            var parent = Parent;
            while (parent is not null)
            {
                p += new Vector2(parent.Left, parent.Top);
                parent = parent.Parent;
            }
            return p;
        }
    }

    // ── State flags ─────────────────────────────────────────────────────
    private bool _visible = true;
    public bool Visible
    {
        get => _visible;
        set
        {
            if (_visible == value) return;

            // A top-level visibility transition is also an ownership boundary:
            // focus/capture/modal state must be released before the subtree becomes
            // unreachable to input. UiRoot filters these notifications through the
            // registered-window manager, while ordinary child visibility changes
            // remain cheap and behavior-neutral.
            UiRoot? root = FindRoot();
            root?.OnElementVisibilityChanging(this, value);
            _visible = value;
            root?.OnElementVisibilityChanged(this, value);
        }
    }
    private bool _enabled = true;
    public bool Enabled
    {
        get => _enabled;
        set
        {
            if (_enabled == value) return;
            _enabled = value;
            OnEnabledChanged();
        }
    }

    /// <summary>
    /// If true, <see cref="HitTest"/> skips this element — the event
    /// passes through to whatever is behind. Used by decoration widgets
    /// (portrait frames, ornamental dividers).
    /// </summary>
    public bool ClickThrough    { get; set; }

    /// <summary>
    /// Optional live visibility reader, evaluated once per tick. Markup
    /// <c>visible="{Binding}"</c> uses this so a panel can show and hide itself
    /// from its binding object's state without the owner touching UI objects.
    /// </summary>
    public Func<bool>? VisibleSource { get; set; }

    /// <summary>
    /// If true, <see cref="UiRoot"/> will set focus here on click,
    /// routing WM_KEYDOWN / WM_CHAR to <see cref="OnEvent"/> as
    /// <see cref="UiEventType.KeyDown"/> / <see cref="UiEventType.Char"/>.
    /// </summary>
    public bool AcceptsFocus    { get; set; }

    /// <summary>
    /// True if this is a text-entry (edit box); used by focus routing
    /// to suppress global hotkeys while typing.
    /// </summary>
    public bool IsEditControl   { get; set; }

    private int _zOrder;

    /// <summary>Painter's-algorithm z-order within siblings. Higher = on top.</summary>
    public int ZOrder
    {
        get => _zOrder;
        set
        {
            if (_zOrder == value) return;
            _zOrder = value;
            Parent?.InvalidateChildOrder();
        }
    }

    /// <summary>Window opacity (0..1) multiplied into this element's and its
    /// descendants' background + sprite draws (text stays opaque). 1 = fully opaque.
    /// Set on a top-level window (e.g. the chat frame) for retail's translucent chat.</summary>
    public float Opacity        { get; set; } = 1f;

    /// <summary>If true, a left-drag on this element (or a non-draggable child of
    /// it) repositions it as a movable window. Intended for top-level panels,
    /// whose Left/Top are screen coordinates (Root sits at the origin).</summary>
    public bool Draggable { get; set; }

    /// <summary>Authored window-move handle (retail <c>UIElement_Dragbar</c>, element
    /// class 2, <c>Register @ 0x0046C840</c>): a left-press inside this element's subtree
    /// moves its top-level window even when that window is not whole-surface
    /// <see cref="Draggable"/> (retail <c>StartMouseMoving @ 0x0046C760</c> calls
    /// <c>UIElement::StartMovement</c> on the parent window). Set by the DAT widget
    /// factory for Type-2 layout elements — e.g. the 600 x 5 strip along the top of the
    /// combat/spell bar. Hovering it shows the window-move cursor.</summary>
    public bool WindowMoveHandle { get; set; }

    /// <summary>Clamp a dragged top-level window fully inside its parent. Retail
    /// <c>gmRadarUI::MoveTo</c> enables this; most windows retain the toolkit default.</summary>
    public bool ConstrainDragToParent { get; set; }

    /// <summary>
    /// Clamp resize growth to the current parent extent. This is distinct from
    /// <see cref="ConstrainDragToParent"/>: retail's shared main-panel host may be
    /// moved freely, but its bottom resizebar grows only to the desktop edge.
    /// </summary>
    public bool ConstrainResizeToParent { get; set; }

    /// <summary>If true, a left-drag starting near this element's edge/corner
    /// resizes it (window resize). Intended for top-level panels.</summary>
    public bool Resizable { get; set; }

    /// <summary>If true, a left-drag starting on this element is delivered to the
    /// element (e.g. text selection) instead of moving/resizing an ancestor window.
    /// Edge resize on a resizable ancestor still wins — only the interior move /
    /// drag-drop candidacy is suppressed in favour of the element's own handling.</summary>
    public bool CapturesPointerDrag { get; set; }

    /// <summary>If true, a left-press-and-move on this element starts a DRAG-DROP
    /// (<see cref="UiRoot"/> promotes to BeginDrag) rather than moving a Draggable
    /// ancestor window — so an item cell inside the toolbar frame drags the item, not
    /// the window. Distinct from <see cref="CapturesPointerDrag"/> (a self-driven
    /// interior drag like text selection, which does NOT promote to BeginDrag). Default
    /// false; overridden by drag sources (e.g. an occupied <see cref="UiItemSlot"/>).</summary>
    public virtual bool IsDragSource => false;

    /// <summary>If true, a left-press on this element is handled BY the element (it receives the Click
    /// on release) instead of being captured as a whole-window move on a Draggable ancestor. Set by
    /// interactive leaf widgets (e.g. <see cref="UiButton"/>) so they stay clickable inside a
    /// whole-window-Draggable frame like the inventory window — where, without this, the IA-12
    /// whole-window-drag swallows the press and the Click is never emitted. Distinct from
    /// <see cref="IsDragSource"/> (starts a drag-drop) and <see cref="CapturesPointerDrag"/> (a
    /// self-driven interior drag such as text selection). Default false.</summary>
    public virtual bool HandlesClick => false;

    /// <summary>
    /// Whether pointer movement should continue reaching this element while it remains
    /// hovered and no mouse capture is active. Composite controls use this to update
    /// procedural sub-region hover state without broadening mouse-move delivery to every
    /// retained widget.
    /// </summary>
    public virtual bool ReceivesHoverMouseMove => false;

    /// <summary>Minimum size enforced while resizing.</summary>
    public float MinWidth { get; set; } = 40f;
    public float MinHeight { get; set; } = 40f;

    /// <summary>Maximum size enforced while resizing (default unbounded).</summary>
    public float MaxWidth { get; set; } = float.MaxValue;
    public float MaxHeight { get; set; } = float.MaxValue;

    /// <summary>Allow horizontal (width) resize. Ignored unless <see cref="Resizable"/>.</summary>
    public bool ResizeX { get; set; } = true;
    /// <summary>Allow vertical (height) resize. Ignored unless <see cref="Resizable"/>.</summary>
    public bool ResizeY { get; set; } = true;

    /// <summary>Which edges may start a resize, beyond the ResizeX/ResizeY axis gates. Default: all.
    /// Set to e.g. <see cref="ResizeEdges.Bottom"/> to allow only a bottom-edge drag (the collapse toolbar).</summary>
    public ResizeEdges ResizableEdges { get; set; } =
        ResizeEdges.Left | ResizeEdges.Right | ResizeEdges.Top | ResizeEdges.Bottom;

    /// <summary>Edges this element anchors to in its parent. Default Left|Top
    /// (pinned top-left, fixed size — no reflow). Left|Right stretches width.</summary>
    private AnchorEdges _anchors = AnchorEdges.Left | AnchorEdges.Top;

    /// <summary>Edges this programmatic element anchors to in its parent. Assigning
    /// this compatibility policy explicitly opts out of an imported DAT
    /// <see cref="LayoutPolicy"/>.</summary>
    public AnchorEdges Anchors
    {
        get => _anchors;
        set
        {
            _anchors = value;
            LayoutPolicy = null;
            _anchorCaptured = false;
        }
    }

    /// <summary>
    /// Exact raw-edge policy for imported DAT elements. Null for roots and
    /// programmatic widgets. Controllers may set null to opt out or call
    /// <see cref="UiLayoutPolicy.Rebase"/> after intentional geometry changes.
    /// </summary>
    public UiLayoutPolicy? LayoutPolicy { get; set; }

    // ── Tree structure ──────────────────────────────────────────────────
    public UiElement? Parent { get; private set; }

    private readonly List<UiElement> _children = new();
    private UiElement[]? _childrenBackToFront;
    private UiElement[]? _childrenFrontToBack;
    public IReadOnlyList<UiElement> Children => _children;

    public virtual void AddChild(UiElement child)
    {
        if (child.Parent is not null) child.Parent.RemoveChild(child);
        child.Parent = this;
        _children.Add(child);
        InvalidateChildOrder();
    }

    /// <summary>
    /// Retail's <c>GetChildRecursive</c>: a depth-first search of
    /// <paramref name="root"/> and every descendant (not just direct
    /// children) for a widget carrying <paramref name="datElementId"/>.
    /// Shared by <see cref="UiTabPanel"/>'s tab-table resolution and any
    /// controller that needs to resolve an element that may be
    /// DUPLICATED (same numeric id) across sibling subtrees — e.g. the
    /// Options panel's Apply/Reset/Defaults buttons, which the
    /// Character/Chat/Config pages each author under the SAME ids
    /// (Campaign OP research doc §3.1), so a flat
    /// <see cref="ImportedLayout.FindElement"/> id lookup over the whole
    /// panel tree cannot reliably pick the right page's own instance —
    /// scoping the search to one page's subtree root can.
    /// </summary>
    public static UiElement? FindDescendant(UiElement root, uint datElementId)
    {
        ArgumentNullException.ThrowIfNull(root);
        if (root.DatElementId == datElementId) return root;
        foreach (UiElement child in root.Children)
        {
            UiElement? found = FindDescendant(child, datElementId);
            if (found is not null) return found;
        }
        return null;
    }

    public virtual bool RemoveChild(UiElement child)
    {
        if (!_children.Contains(child)) return false;
        FindRoot()?.OnSubtreeRemoving(child);
        _children.Remove(child);
        child.Parent = null;
        InvalidateChildOrder();
        return true;
    }

    /// <summary>
    /// Stable snapshots of the current sibling order. Retained UI mutation is
    /// render-thread-owned, so the arrays can be reused until membership or a
    /// child's Z-order changes. A traversal keeps its local array if a callback
    /// mutates the tree, preserving the prior snapshot semantics without a
    /// per-element allocation on every draw and overlay pass.
    /// </summary>
    internal UiElement[] ChildrenBackToFrontSnapshot()
    {
        if (_childrenBackToFront is not null)
            return _childrenBackToFront;

        _childrenBackToFront = _children.ToArray();
        Array.Sort(
            _childrenBackToFront,
            static (a, b) => a.ZOrder.CompareTo(b.ZOrder));
        return _childrenBackToFront;
    }

    internal UiElement[] ChildrenFrontToBackSnapshot()
    {
        if (_childrenFrontToBack is not null)
            return _childrenFrontToBack;

        UiElement[] backToFront = ChildrenBackToFrontSnapshot();
        _childrenFrontToBack = new UiElement[backToFront.Length];
        for (int source = backToFront.Length - 1, destination = 0;
             source >= 0;
             source--, destination++)
        {
            _childrenFrontToBack[destination] = backToFront[source];
        }

        return _childrenFrontToBack;
    }

    private void InvalidateChildOrder()
    {
        _childrenBackToFront = null;
        _childrenFrontToBack = null;
    }

    /// <summary>
    /// True if this widget draws its full appearance itself and REPRODUCES its dat
    /// sub-elements procedurally (3-slice caps, button labels, scroll arrows, popup
    /// rows…) — so the <see cref="AcDream.App.UI.Layout.LayoutImporter"/> must NOT build
    /// those dat child elements as separate widgets (they would double-draw and, worse,
    /// steal pointer/focus from the behavioral widget). All registered behavioral widgets
    /// (Meter/Menu/Button/Scrollbar/Text/Field) return <c>true</c>; the generic container
    /// (<see cref="AcDream.App.UI.Layout.UiDatElement"/>) and panels return <c>false</c>
    /// and recurse their children normally. Mirrors retail, where each
    /// <c>UIElement_X::DrawSelf</c> owns its internal structure.
    /// </summary>
    public virtual bool ConsumesDatChildren => false;

    // ── Virtual overrides ───────────────────────────────────────────────

    /// <summary>
    /// Draw THIS element (not its children). Children are composited by
    /// <see cref="UiRoot"/> after this returns.
    /// </summary>
    protected virtual void OnDraw(UiRenderContext ctx) { }

    /// <summary>
    /// Draw AFTER this element's own children, but still within this element's
    /// transform/alpha (NOT a global pass like <see cref="OnDrawOverlay"/>). Use for a
    /// window FRAME border, which must be the outermost layer drawn OVER its content's
    /// edges (so content can't poke through the frame), while the frame's center fill
    /// stays a background in <see cref="OnDraw"/>. Default: nothing.
    /// </summary>
    protected virtual void OnDrawAfterChildren(UiRenderContext ctx) { }

    /// <summary>
    /// Draw content that must sit ON TOP of the ENTIRE UI, regardless of this
    /// element's position in the tree — open menus, dropdowns, tooltips. Called in
    /// a SECOND traversal after the whole tree's <see cref="OnDraw"/> pass, with the
    /// same accumulated transform/alpha this element had during its normal draw.
    /// Retail spawns popups as ROOT elements (UIElement_Menu::MakePopup) for exactly
    /// this reason; this is the equivalent without reparenting. Default: nothing.
    /// </summary>
    protected virtual void OnDrawOverlay(UiRenderContext ctx) { }

    /// <summary>
    /// When true, descendant drawing and hit-testing are clipped to this element's
    /// local bounds. Scrollable listboxes use this so edge rows can remain visible
    /// at arbitrary pixel offsets without painting or receiving input outside the viewport.
    /// </summary>
    protected virtual bool ClipsChildren => false;

    /// <summary>Per-frame tick (animations, timers, caret blink).</summary>
    protected virtual void OnTick(double deltaSeconds) { }

    /// <summary>Called synchronously after <see cref="Enabled"/> changes.</summary>
    protected virtual void OnEnabledChanged() { }

    /// <summary>
    /// Custom hit-test override. Default is a rectangle containment
    /// check on (<see cref="Width"/>, <see cref="Height"/>).
    /// </summary>
    protected virtual bool OnHitTest(float localX, float localY)
        => localX >= 0f && localX < Width && localY >= 0f && localY < Height;

    /// <summary>
    /// Event handler. Return <c>true</c> to consume the event (the
    /// <see cref="UiRoot"/> will stop propagation). Return <c>false</c>
    /// to let ancestors / fall-through handle it.
    /// </summary>
    public virtual bool OnEvent(in UiEvent e) => false;

    /// <summary>The data this element carries when a drag begins. <see cref="UiRoot"/>
    /// pulls this on drag-promote; a NULL return CANCELS the drag (retail:
    /// ItemList_BeginDrag only arms an occupied cell). Default null = not draggable.</summary>
    public virtual object? GetDragPayload() => null;

    /// <summary>The texture <see cref="UiRoot"/> paints at the cursor while this element
    /// is the drag source: (GL handle, width, height). Null = no ghost. Keeps
    /// <see cref="UiRoot"/> item-agnostic. Retail analog: m_dragIcon (decomp 229738).</summary>
    public virtual (uint tex, int w, int h)? GetDragGhost() => null;

    /// <summary>
    /// Notifies the source widget when the root starts or finishes carrying its drag payload.
    /// This is a retained-widget lifecycle hook rather than item-specific root logic. Retail's
    /// item implementation uses it to show/hide <c>m_elem_Icon_Ghosted</c> around a physical
    /// item drag (<c>ItemList_BeginDrag</c> 0x004e32d0; <c>SetWaitingState</c> 0x004e11b0).
    /// </summary>
    internal virtual void SetDragSourceActive(bool active, object? payload) { }

    /// <summary>
    /// Tooltip text for this widget. Retail fires event 0x07 after the configured
    /// hover delay (0.25 seconds by default), then queries the widget's virtual "GetString"
    /// (vtable +0x88) to render the tooltip body.
    /// </summary>
    public virtual string? GetTooltipText() => null;

    // ── Framework entry points (internal, called by UiRoot) ─────────────

    internal void DrawSelfAndChildren(UiRenderContext ctx)
    {
        if (!Visible) return;

        // Translate into our local space + push this window's opacity (multiplies into
        // descendants' sprite, rect, AND text draws — CH6c ported DrawStringDat/DrawString
        // through the same ApplyAlpha chokepoint as sprites/rects, matching retail's
        // ChatInterface::SetOpacity (0x004F3120), which fades the whole composited window
        // surface, chrome and glyphs together, not text-stays-sharp over a translucent panel).
        ctx.PushTransform(Left, Top);
        ctx.PushAlpha(Opacity);
        try
        {
            OnDraw(ctx);

            // Anchor layout: reflow children to this element's current size.
            for (int i = 0; i < _children.Count; i++)
                _children[i].ApplyAnchor(Width, Height);

            // Children painted back-to-front (lowest ZOrder first).
            if (_children.Count > 0)
            {
                bool clipsChildren = ClipsChildren;
                if (clipsChildren)
                    ctx.PushClip(0f, 0f, Width, Height);
                try
                {
                    UiElement[] ordered = ChildrenBackToFrontSnapshot();
                    for (int i = 0; i < ordered.Length; i++)
                        ordered[i].DrawSelfAndChildren(ctx);
                }
                finally
                {
                    if (clipsChildren)
                        ctx.PopClip();
                }
            }

            // Foreground pass for this element (e.g. a window frame's border drawn
            // OVER its content's edges). Default no-op for ordinary elements.
            OnDrawAfterChildren(ctx);
        }
        finally
        {
            ctx.PopAlpha();
            ctx.PopTransform();
        }
    }

    /// <summary>Second draw traversal: re-walks the tree applying the same
    /// transform/alpha as <see cref="DrawSelfAndChildren"/> and calls
    /// <see cref="OnDrawOverlay"/> on each element, so popups composite on top of
    /// everything drawn in the main pass (dat-font glyphs and sprites share one
    /// submission-ordered bucket, so later submissions win).</summary>
    internal void DrawOverlays(UiRenderContext ctx)
    {
        if (!Visible) return;
        ctx.PushTransform(Left, Top);
        ctx.PushAlpha(Opacity);
        try
        {
            OnDrawOverlay(ctx);
            if (_children.Count > 0)
            {
                bool clipsChildren = ClipsChildren;
                if (clipsChildren)
                    ctx.PushClip(0f, 0f, Width, Height);
                try
                {
                    UiElement[] ordered = ChildrenBackToFrontSnapshot();
                    for (int i = 0; i < ordered.Length; i++)
                        ordered[i].DrawOverlays(ctx);
                }
                finally
                {
                    if (clipsChildren)
                        ctx.PopClip();
                }
            }
        }
        finally
        {
            ctx.PopAlpha();
            ctx.PopTransform();
        }
    }

    internal void TickSelfAndChildren(double dt)
    {
        // Evaluated before the Visible gate on purpose: the gate returns early
        // for a hidden element, so a source read after it could turn an element
        // off but never back on.
        if (VisibleSource is { } visibility)
            Visible = visibility();
        if (!Visible) return;
        OnTick(dt);
        for (int i = 0; i < _children.Count; i++)
            _children[i].TickSelfAndChildren(dt);
    }

    /// <summary>
    /// Top-down, children-first hit-test. <paramref name="localX"/> /
    /// <paramref name="localY"/> are in THIS element's local space.
    /// Returns the topmost descendant (or this) at the point, or null.
    /// </summary>
    internal UiElement? HitTest(float localX, float localY)
    {
        if (!Visible || !Enabled) return null;
        if (ClipsChildren
            && (localX < 0f || localX >= Width || localY < 0f || localY >= Height))
            return null;

        // Children first, in reverse Z-order (topmost first). ClickThrough means
        // THIS element is transparent to the pointer — but its children are NOT.
        // A ClickThrough container (e.g. a UiDatElement panel that hosts the chat
        // input / transcript) must still let the pointer reach its behavioral
        // children, so the ClickThrough check happens AFTER the child walk, gating
        // only whether THIS element claims the hit.
        if (_children.Count > 0)
        {
            UiElement[] ordered = ChildrenFrontToBackSnapshot();
            for (int i = 0; i < ordered.Length; i++)
            {
                var c = ordered[i];
                var childHit = c.HitTest(localX - c.Left, localY - c.Top);
                if (childHit is not null) return childHit;
            }
        }

        if (ClickThrough) return null;
        return OnHitTest(localX, localY) ? this : null;
    }

    // ── Anchor layout ────────────────────────────────────────────────────

    private bool _anchorCaptured;
    private float _amL, _amT, _amR, _amB, _aw0, _ah0;

    /// <summary>Reposition/resize this element per <see cref="Anchors"/>, keeping
    /// the margins captured (at first layout / design size) to each anchored edge.
    /// Called by the parent each frame before drawing children.</summary>
    internal void ApplyAnchor(float parentW, float parentH)
    {
        if (LayoutPolicy is not null)
        {
            var current = UiPixelRect.FromPositionAndSize(
                (int)Left,
                (int)Top,
                (int)Width,
                (int)Height);
            var parent = UiPixelRect.FromPositionAndSize(0, 0, (int)parentW, (int)parentH);
            var next = LayoutPolicy.Apply(current, parent);
            Left = next.X0;
            Top = next.Y0;
            Width = next.Width;
            Height = next.Height;
            return;
        }

        if (Anchors == AnchorEdges.None) return;
        if (!_anchorCaptured)
        {
            _amL = Left; _amT = Top;
            _amR = parentW - (Left + Width);
            _amB = parentH - (Top + Height);
            _aw0 = Width; _ah0 = Height;
            _anchorCaptured = true;
        }
        var (x, y, w, h) = ComputeAnchoredRect(Anchors, _amL, _amT, _amR, _amB, _aw0, _ah0, parentW, parentH);
        Left = x; Top = y; Width = w; Height = h;
    }

    /// <summary>
    /// Make the current geometry the new layout baseline after an intentional
    /// controller/runtime change. Compatibility anchors recapture their margins on
    /// the next layout pass; imported raw-edge policies rebase immediately against
    /// the current parent rect.
    /// </summary>
    internal void ResetAnchorCapture()
    {
        _anchorCaptured = false;
        if (LayoutPolicy is null || Parent is null) return;

        LayoutPolicy.Rebase(
            UiPixelRect.FromPositionAndSize(
                (int)Left,
                (int)Top,
                (int)Width,
                (int)Height),
            UiPixelRect.FromPositionAndSize(
                0,
                0,
                (int)Parent.Width,
                (int)Parent.Height));
    }

    /// <summary>
    /// Capture the current mounted rect as the baseline for a compatibility
    /// anchor policy immediately. Hidden trees do not receive a layout traversal,
    /// so lazy capture after a persistence resize would use the wrong parent size.
    /// </summary>
    internal void CaptureCurrentAnchorBaseline()
    {
        if (Parent is null || Anchors == AnchorEdges.None) return;
        _anchorCaptured = false;
        ApplyAnchor(Parent.Width, Parent.Height);
    }

    /// <summary>
    /// Rebase each immediate child after a host deliberately changes this element's
    /// design extent. This is used when a production DAT root contains auxiliary
    /// space that is cropped by its retail window mount; descendants must reflow
    /// from the mounted content rect, not from the uncropped LayoutDesc display box.
    /// </summary>
    internal void RebaseChildLayoutBaselines()
    {
        foreach (var child in _children)
            child.ResetAnchorCapture();
    }

    /// <summary>Walk up to the owning <see cref="UiRoot"/> (the top of the tree), or null
    /// if this element is not attached. Lets a widget reach focus/capture services — e.g.
    /// a chat input blurring itself (exiting write mode) after submit.</summary>
    internal UiRoot? FindRoot()
    {
        UiElement e = this;
        while (e.Parent is not null) e = e.Parent;
        return e as UiRoot;
    }

    /// <summary>Compute an anchored child rect. Left&amp;Right ⇒ stretch width
    /// (keep both margins); Right only ⇒ pin to right at fixed width; otherwise
    /// pin left at fixed width. Same logic vertically.</summary>
    public static (float x, float y, float w, float h) ComputeAnchoredRect(
        AnchorEdges a, float mL, float mT, float mR, float mB,
        float w0, float h0, float parentW, float parentH)
    {
        bool l = (a & AnchorEdges.Left) != 0, r = (a & AnchorEdges.Right) != 0;
        float x, w;
        if (l && r) { x = mL; w = parentW - mR - mL; }
        else if (r) { w = w0; x = parentW - mR - w0; }
        else { x = mL; w = w0; }

        bool t = (a & AnchorEdges.Top) != 0, b = (a & AnchorEdges.Bottom) != 0;
        float y, h;
        if (t && b) { y = mT; h = parentH - mB - mT; }
        else if (b) { h = h0; y = parentH - mB - h0; }
        else { y = mT; h = h0; }

        if (w < 0) w = 0;
        if (h < 0) h = 0;
        return (x, y, w, h);
    }
}