using System;
using System.Collections.Generic;
using DatReaderWriter;
using DatReaderWriter.DBObjs;
using DatReaderWriter.Enums;
using DatReaderWriter.Types;

namespace AcDream.App.UI.Layout;

/// <summary>
/// The result of importing a retail LayoutDesc: a <see cref="UiElement"/> tree with
/// an O(1) lookup table for finding any element by its dat id.
/// </summary>
public sealed class ImportedLayout
{
    /// <summary>Root widget of the imported tree.</summary>
    public UiElement Root { get; }

    private readonly Dictionary<uint, UiElement> _byId;

    public ImportedLayout(UiElement root, Dictionary<uint, UiElement> byId)
    {
        Root  = root;
        _byId = byId;
    }

    /// <summary>Find a widget by its dat element id (e.g. <c>0x100000E6</c>).
    /// Returns null if the id was skipped (Type-12 prototype) or not present.</summary>
    public UiElement? FindElement(uint id)
        => _byId.TryGetValue(id, out var e) ? e : null;
}

/// <summary>
/// Two-layer layout importer for retail LayoutDesc dat objects.
///
/// <para>
/// <strong>Pure layer</strong> (<see cref="Build"/> / <see cref="BuildFromInfos"/>):
/// converts a pre-resolved <see cref="ElementInfo"/> tree into a <see cref="UiElement"/>
/// tree via <see cref="DatWidgetFactory"/>. Testable without dats or OpenGL — all tests
/// in <c>LayoutImporterTests.cs</c> exercise this layer only.
/// </para>
///
/// <para>
/// <strong>Dat shell</strong> (<see cref="Import"/>): reads a <see cref="LayoutDesc"/>,
/// converts each top-level <see cref="ElementDesc"/> to a fully resolved
/// <see cref="ElementInfo"/> (applying <c>BaseElement</c> / <c>BaseLayoutId</c>
/// inheritance with a cycle guard), then delegates to <see cref="Build"/>.
/// </para>
///
/// <para>
/// Meter elements (Type 7) consume their own dat-children: <see cref="DatWidgetFactory"/>
/// reads the grandchild slice-sprite ids during <see cref="UiMeter"/> construction, so the
/// children must NOT be added as separate <see cref="UiElement"/> nodes in the tree.
/// Every other element type recurses its children generically.
/// </para>
/// </summary>
public static class LayoutImporter
{
    // ── Pure layer ────────────────────────────────────────────────────────────

    /// <summary>
    /// Convenience for tests: attach <paramref name="children"/> to
    /// <paramref name="rootInfo"/>, then call <see cref="Build"/>.
    /// The children list is set directly on <paramref name="rootInfo"/>;
    /// any existing children are replaced.
    /// </summary>
    public static ImportedLayout BuildFromInfos(
        ElementInfo rootInfo,
        IEnumerable<ElementInfo> children,
        Func<uint, (uint, int, int)> resolve,
        UiDatFont? datFont,
        Func<uint, UiDatFont?>? fontResolve = null)
    {
        rootInfo.Children = new List<ElementInfo>(children);
        return Build(rootInfo, resolve, datFont, fontResolve);
    }

    /// <summary>
    /// Pure builder: produce the widget tree from a fully resolved
    /// <see cref="ElementInfo"/> tree (children already attached).
    /// </summary>
    /// <param name="fontResolve">Optional per-element font resolver — FontDid →
    /// <see cref="UiDatFont"/> (or null if the font can't be loaded). When supplied,
    /// elements with a non-zero <see cref="ElementInfo.FontDid"/> get their own dat
    /// font at build time instead of the shared <paramref name="datFont"/> fallback.
    /// Null preserves the original single-font behavior for all callers that don't
    /// pass it — no behavior change for the live game path.</param>
    public static ImportedLayout Build(
        ElementInfo rootInfo,
        Func<uint, (uint, int, int)> resolve,
        UiDatFont? datFont,
        Func<uint, UiDatFont?>? fontResolve = null)
    {
        var byId = new Dictionary<uint, UiElement>();
        // Root is never a Type-12 prototype in practice; fall back to a generic
        // container if the factory returns null for an exotic root type.
        var root = BuildWidget(rootInfo, resolve, datFont, fontResolve, byId);
        if (root is null)
        {
            Console.WriteLine($"[D.2b] LayoutImporter: root element 0x{rootInfo.Id:X8} (type {rootInfo.Type}) produced no widget — using empty container fallback.");
            root = new UiDatElement(rootInfo, resolve);
        }
        return new ImportedLayout(root, byId);
    }

    private static UiElement? BuildWidget(
        ElementInfo info,
        Func<uint, (uint, int, int)> resolve,
        UiDatFont? datFont,
        Func<uint, UiDatFont?>? fontResolve,
        Dictionary<uint, UiElement> byId)
    {
        var w = DatWidgetFactory.Create(info, resolve, datFont, fontResolve);
        if (w is null) return null;               // Type-12 style prototype — skip

        if (info.Id != 0) byId[info.Id] = w;

        // Behavioral widgets that draw their full appearance + reproduce their dat
        // sub-elements procedurally (Meter's 3-slice, Menu's label/rows, Field/Text caps,
        // Button labels, Scrollbar arrows) CONSUME their dat children — building those as
        // separate widgets double-draws and lets an invisible child steal pointer/focus
        // from the behavioral widget (e.g. the channel Menu's label child intercepting the
        // button click). Only generic containers (UiDatElement, panels) recurse. See
        // UiElement.ConsumesDatChildren.
        if (!w.ConsumesDatChildren)
        {
            foreach (var child in info.Children)
            {
                var cw = BuildWidget(child, resolve, datFont, fontResolve, byId);
                if (cw is not null) w.AddChild(cw);
            }
        }
        else if (w is UiMeter)
        {
            // Fix 5: UiMeter.ConsumesDatChildren=true swallows ALL children, including text
            // label/value overlays that are separate renderable widgets (not part of the bar
            // art). BuildMeter in DatWidgetFactory already consumed the Type-3 slice containers
            // (reads their grandchild sprite ids to populate Back*/Front* properties). The
            // remaining non-Type-3 children (typically Type-12 UIElement_Text overlays such
            // as the XP meter's 0x10000237 label + 0x10000238 value) ARE renderable and belong
            // in the widget tree. We build them here explicitly, registered in byId so
            // FindElement can locate them, and attached as children of the meter so they render
            // as overlays at their dat-local coordinates. The controller can then locate these
            // widgets via FindElement and bind LinesProvider without injecting new runtime nodes.
            //
            // Type-3 children are SKIPPED here because BuildMeter already consumed them (they
            // carry the 3-slice sprite ids, not text content; building them again would
            // double-draw the bar art). All other child types are built normally.
            //
            // Safe for vitals: the health/stamina/mana meters have ONLY Type-3 slice children
            // (no text children). This loop finds nothing for them → no change to vitals.
            foreach (var child in info.Children)
            {
                if (child.Type == 3) continue;   // slice containers: already consumed by BuildMeter
                var cw = BuildWidget(child, resolve, datFont, fontResolve, byId);
                if (cw is not null) w.AddChild(cw);
            }
        }

        return w;
    }

    // ── Dat shell ─────────────────────────────────────────────────────────────

    /// <summary>
    /// Dat shell, ElementInfo half: load the layout + resolve inheritance + build the
    /// ElementInfo tree (no widgets). Exposed for fixture generation + conformance tests.
    /// Returns null if the layout is missing.
    /// </summary>
    /// <param name="dats">The dat collection to read the LayoutDesc from.</param>
    /// <param name="layoutId">The LayoutDesc dat id to read.</param>
    public static ElementInfo? ImportInfos(DatCollection dats, uint layoutId)
    {
        var ld = dats.Get<LayoutDesc>(layoutId);
        if (ld is null) return null;

        // Collect the set of element ids that are referenced as a BaseElement by ANY
        // element in THIS layout (where BaseLayoutId == layoutId). Such elements are
        // purely inheritance templates ("prototypes") — retail never instantiates them
        // as live widgets. Example: the toolbar slot prototype 0x100001B2 in LayoutDesc
        // 0x21000016, which all 18 slot elements inherit from and which has no own media.
        //
        // NOTE: the Resolve path reads BaseElement from the raw dat directly (via
        // dats.Get<LayoutDesc>), so the prototype never needs to appear in the built
        // widget tree for inheritance to work. Skipping it here is safe.
        var referencedAsBase = new HashSet<uint>();
        foreach (var kv in ld.Elements)
            CollectBaseRefsInDesc(kv.Value, layoutId, referencedAsBase);

        var tops = new List<ElementInfo>();
        foreach (var kv in ld.Elements)
        {
            // Skip pure prototype elements: top-level elements that are referenced as a
            // base template by another element in this same layout AND have no own state
            // media (so they draw nothing and contribute nothing but their inherited shape).
            var d = kv.Value;
            if (referencedAsBase.Contains(d.ElementId) && HasNoOwnMedia(d))
            {
                Console.WriteLine($"[D.2b] LayoutImporter: skipping prototype element 0x{d.ElementId:X8} in layout 0x{layoutId:X8} (no own media, referenced as BaseElement).");
                continue;
            }

            tops.Add(Resolve(dats, d, new HashSet<(uint, uint)>()));
        }

        return tops.Count == 1
            ? tops[0]
            : new ElementInfo { Id = 0, Type = 3, Children = tops };
    }

    /// <summary>
    /// Dat shell: load the LayoutDesc, resolve inheritance for every top-level
    /// element, and build the widget tree. Returns null if the layout is absent
    /// from the dats.
    ///
    /// <para>
    /// <b>Dat UIState visibility model (2026-06-26 audit):</b>
    /// The dat's <c>ElementDesc.DefaultState</c> field specifies which SPRITE/MEDIA
    /// state an element starts in (e.g., <c>Normal</c>, <c>Minimized</c>). It does
    /// NOT encode visibility of sibling Group containers. The <c>StateDesc</c>'s
    /// <see cref="DatReaderWriter.Enums.IncorporationFlags"/> contains X/Y/Width/Height/
    /// ZLevel/PassToChildren — there is no Visible flag.
    /// </para>
    ///
    /// <para>
    /// Windows that display multiple sibling Group containers at the same position
    /// (the character footer's three state-groups; the tab-page content areas) manage
    /// visibility purely at runtime via C++ controller code. Retail uses
    /// <c>UIElement::SetState(stateId)</c> on the parent to propagate state, then
    /// C++ getters access the right sub-group by element id. All groups are shipped
    /// as visible in the imported widget tree; the relevant controllers
    /// (<see cref="CharacterStatController"/>) perform the initial show/hide.
    /// </para>
    /// </summary>
    /// <param name="fontResolve">Optional per-element font resolver (see
    /// <see cref="Build"/> for details). Null = original single-font behavior.</param>
    public static ImportedLayout? Import(
        DatCollection dats,
        uint layoutId,
        Func<uint, (uint, int, int)> resolve,
        UiDatFont? datFont,
        Func<uint, UiDatFont?>? fontResolve = null)
    {
        var rootInfo = ImportInfos(dats, layoutId);
        if (rootInfo is null) return null;
        return Build(rootInfo, resolve, datFont, fontResolve);
    }

    // ── Inheritance resolution ────────────────────────────────────────────────

    /// <summary>True when a pure-container leaf should inherit its base's subtree (the
    /// gmInventoryUI sub-window mount): the derived element has no own children, no own
    /// state media, and the base resolved to ≥1 child. Inert for media-bearing inheritors
    /// (close button / title), elements with their own children, and childless style
    /// prototypes (vitals/chat/toolbar text — the base prototype has no children).</summary>
    internal static bool ShouldMountBaseChildren(int derivedChildCount, int derivedMediaCount, int baseChildCount)
        => derivedChildCount == 0 && derivedMediaCount == 0 && baseChildCount > 0;

    /// <summary>
    /// Converts an <see cref="ElementDesc"/> to a resolved <see cref="ElementInfo"/>:
    /// reads own fields + media, applies the BaseElement / BaseLayoutId chain
    /// (cycle-guarded by <paramref name="baseChain"/>), then resolves + attaches children.
    /// </summary>
    private static ElementInfo Resolve(
        DatCollection dats,
        ElementDesc d,
        HashSet<(uint layoutId, uint elementId)> baseChain)
    {
        // Read this element's own fields + media (no inheritance, no children yet).
        var self = ToInfo(d);
        var result = self;
        List<ElementInfo>? baseChildren = null;

        // Apply BaseElement / BaseLayoutId inheritance if present.
        if (d.BaseElement != 0 && d.BaseLayoutId != 0
            && baseChain.Add((d.BaseLayoutId, d.BaseElement)))
        {
            var baseLd   = dats.Get<LayoutDesc>(d.BaseLayoutId);
            var baseDesc = baseLd is null ? null : FindDesc(baseLd, d.BaseElement);
            if (baseDesc is not null)
            {
                // Recurse the base chain (already guarded by the HashSet add above).
                var baseInfo = Resolve(dats, baseDesc, baseChain);
                // Derived fields override the base; children are attached below.
                result = ElementReader.Merge(baseInfo, self);
                baseChildren = baseInfo.Children;   // capture for the sub-window mount
            }
        }

        // Resolve + attach children.  Each child gets a FRESH base-chain set:
        // the cycle guard is per-element, not shared across siblings.
        foreach (var kv in d.Children)
            result.Children.Add(Resolve(dats, kv.Value, new HashSet<(uint, uint)>()));

        // Sub-window mount: a pure-container leaf (no own children, no own media) that inherits
        // from a base WITH content attaches the base's subtree. Targets the gmInventoryUI panels
        // (0x100001CD/CE/CF — Type-0, media-less, childless, BaseLayoutId → a gm*UI window);
        // inert for media-bearing inheritors (close button/title) and childless style prototypes
        // (vitals/chat/toolbar). See ShouldMountBaseChildren + the B-Grid spec.
        if (baseChildren is not null
            && ShouldMountBaseChildren(d.Children.Count, self.StateMedia.Count, baseChildren.Count))
        {
            result.Children.AddRange(baseChildren);

            // The mounted slot's layer WITHIN THE FRAME is its OWN ZLevel, not the mounted
            // sub-window root's. The gm*UI sub-window roots carry ZLevel 1000 (their standalone
            // top-window layer); ElementReader.Merge's zero-wins-base rule made the slot (own
            // ZLevel 0) inherit that 1000, and the #145 ZOrder fold (ReadOrder − ZLevel·10000)
            // turns 1000 into ZOrder ≈ −10,000,000 — sinking the whole panel BEHIND the frame's
            // Alphablend backdrop (ZLevel 100 → ≈ −1,000,000). The backdrop then overpaints the
            // panel's captions/meter/cells (the wash-out bug; the paperdoll root happens to be
            // ZLevel 0 so it escaped). Restore the slot's own frame-layer so the panel sits in
            // FRONT of the backdrop. (B-Controller debug 2026-06-21; continuation of #145.)
            result.ZLevel = self.ZLevel;

            // Sub-layout slot sizing: when a sub-layout is mounted into a slot that is TALLER
            // (or wider) than the sub-layout's own design height, the cascade of Bottom/Right
            // anchored elements must be resized to fill the slot. Without this step, the
            // background element (0x10000226 for the Attributes tab, designed at 337px) captures
            // _amB = slotH - designH = 238 and permanently stays at 337px, and every element
            // within it with Bottom anchor stays at its design size too (list box at 160px
            // instead of the correct 398px).
            //
            // Retail reference: UIElement::UpdateForParentSizeChange (C++ runtime) propagates a
            // new parent height down the whole tree, updating each Bottom-anchored child's
            // rect by maintaining its bottom margin.  We replicate that cascade here at import
            // time on the base-children subtree so the anchor-capture during the first render
            // frame sees zero (or unchanged) margins — not the spurious "slot bigger than design"
            // margin.
            //
            // Safe guard: only fire when the slot has explicit size AND the base children are
            // smaller (i.e., this is the "slot bigger than design" case).
            // Inventory/paperdoll slots are unaffected because their slot H already matches the
            // sub-layout design H, so no cascade occurs.
            if (result.Width > 0 && result.Height > 0)
            {
                foreach (var child in baseChildren)
                {
                    var childAnchors = ElementReader.ToAnchors(child.Left, child.Top, child.Right, child.Bottom);
                    const AnchorEdges StretchAll = AnchorEdges.Left | AnchorEdges.Top | AnchorEdges.Right | AnchorEdges.Bottom;
                    if (child.X == 0 && child.Y == 0
                        && (childAnchors & StretchAll) == StretchAll   // full-stretch background
                        && child.Width <= result.Width
                        && child.Height > 0 && child.Height < result.Height)  // smaller than slot
                    {
                        // Cascade UpdateForParentSizeChange down the subtree: maintain each
                        // Bottom-anchored element's bottom margin while growing the parent height.
                        CascadeHeight(child, child.Height, result.Height);
                    }
                }
            }
        }

        return result;
    }

    /// <summary>
    /// Read an <see cref="ElementDesc"/>'s own scalar fields + state media into a
    /// fresh <see cref="ElementInfo"/>. No inheritance is applied; children are not
    /// attached (the caller handles those).
    /// </summary>
    private static ElementInfo ToInfo(ElementDesc d)
    {
        // Normalize DefaultState: UIStateId.ToString() gives "Undef"/"Undefined" or "0" when
        // no default is set; map those to "" so UiDatElement treats them as "no preference".
        var defState = d.DefaultState.ToString();
        var info = new ElementInfo
        {
            Id               = d.ElementId,
            Type             = d.Type,
            X                = (float)d.X,
            Y                = (float)d.Y,
            Width            = (float)d.Width,
            Height           = (float)d.Height,
            Left             = d.LeftEdge,
            Top              = d.TopEdge,
            Right            = d.RightEdge,
            Bottom           = d.BottomEdge,
            ReadOrder        = d.ReadOrder,
            ZLevel           = d.ZLevel,
            DefaultStateName = (defState is "Undef" or "Undefined" or "0") ? "" : defState,
        };

        // DirectState (unnamed, key "").
        if (d.StateDesc is not null)
            ReadState(d.StateDesc, "", info);

        // Named states (e.g. UIStateId.HideDetail → "HideDetail").
        foreach (var s in d.States)
            ReadState(s.Value, s.Key.ToString(), info);

        return info;
    }

    /// <summary>
    /// Read the first <see cref="MediaDescImage"/> from <paramref name="sd"/> into
    /// <c>info.StateMedia[name]</c> and extract the font DID from property 0x1A
    /// (<c>ArrayBaseProperty → DataIdBaseProperty</c>) if not yet set.
    /// </summary>
    private static void ReadState(StateDesc sd, string name, ElementInfo info)
    {
        // Only MediaDescImage is read for rendering; MediaDescCursor items (on grips/drag bars)
        // are intentionally skipped — cursor behavior is Plan 2.
        foreach (var m in sd.Media)
        {
            if (m is MediaDescImage img && img.File != 0)
            {
                info.StateMedia[name] = (img.File, (int)img.DrawMode);
                break;
            }
        }

        // Font DID: Properties[0x1A] is ArrayBaseProperty{ DataIdBaseProperty }.
        // Format doc §3: "ArrayBaseProperty containing ONE DataIdBaseProperty".
        if (info.FontDid == 0 && sd.Properties is not null
            && sd.Properties.TryGetValue(0x1Au, out var raw)
            && raw is ArrayBaseProperty arr && arr.Value.Count > 0
            && arr.Value[0] is DataIdBaseProperty did)
        {
            info.FontDid = did.Value;
        }

        if (sd.Properties is not null)
        {
            // HorizontalJustification (0x14): EnumBaseProperty.
            // Retail values: 0=Left, 1=Center, 3=Right, 5=Right (treat 5 as Right per spec).
            // Only update if still at the default (Center); derived-wins handled in Merge.
            if (info.HJustify == HJustify.Center
                && sd.Properties.TryGetValue(0x14u, out var hRaw)
                && hRaw is EnumBaseProperty hEnum)
            {
                info.HJustify = hEnum.Value switch
                {
                    0u => HJustify.Left,
                    1u => HJustify.Center,
                    3u => HJustify.Right,
                    5u => HJustify.Right,
                    _  => HJustify.Center,   // unknown → center (safe default)
                };
            }

            // VerticalJustification (0x15): EnumBaseProperty.
            // Retail values: 2=Top, 4=Bottom; absent/other = Center.
            if (info.VJustify == VJustify.Center
                && sd.Properties.TryGetValue(0x15u, out var vRaw)
                && vRaw is EnumBaseProperty vEnum)
            {
                info.VJustify = vEnum.Value switch
                {
                    2u => VJustify.Top,
                    4u => VJustify.Bottom,
                    _  => VJustify.Center,
                };
            }

            // ColorBaseProperty (0x1B): ARGB bytes → normalized [0,1] Vector4 (R,G,B,A).
            // Only read when not already set (first dat state wins; Merge propagates from base).
            if (info.FontColor is null
                && sd.Properties.TryGetValue(0x1Bu, out var cRaw)
                && cRaw is ColorBaseProperty cProp)
            {
                var c = cProp.Value;
                // ColorARGB stores components as bytes (0–255); normalize to [0,1] for Vector4.
                // Alpha=0 in the dat typically means fully opaque (retail convention: 0 → 255).
                float a = c.Alpha == 0 ? 1f : c.Alpha / 255f;
                info.FontColor = new System.Numerics.Vector4(c.Red / 255f, c.Green / 255f, c.Blue / 255f, a);
            }
        }
    }

    // ── Prototype detection helpers ───────────────────────────────────────────

    /// <summary>
    /// Recursively walks <paramref name="d"/> and all its children, adding to
    /// <paramref name="result"/> the <c>BaseElement</c> of every descriptor that
    /// references this layout (<c>BaseLayoutId == layoutId</c>). Used by
    /// <see cref="ImportInfos"/> to identify pure prototype/template elements that
    /// should not be instantiated as live widgets.
    /// </summary>
    private static void CollectBaseRefsInDesc(ElementDesc d, uint layoutId, HashSet<uint> result)
    {
        if (d.BaseElement != 0 && d.BaseLayoutId == layoutId)
            result.Add(d.BaseElement);
        foreach (var kv in d.Children)
            CollectBaseRefsInDesc(kv.Value, layoutId, result);
    }

    /// <summary>
    /// Returns true when <paramref name="d"/> carries no own state media — i.e. its
    /// <c>StateDesc</c> (DirectState) and <c>States</c> (named states) yield no
    /// <see cref="MediaDescImage"/> entries with a non-zero file id.
    /// Such elements are pure inheritance templates with no rendering content.
    /// </summary>
    private static bool HasNoOwnMedia(ElementDesc d)
    {
        // Re-use ToInfo's media extraction: if the resulting StateMedia is empty the
        // element has no renderable image in any state.
        var info = ToInfo(d);
        return info.StateMedia.Count == 0;
    }

    // ── Element tree search ───────────────────────────────────────────────────

    /// <summary>
    /// Find an <see cref="ElementDesc"/> by id anywhere in the top-level tree of
    /// <paramref name="ld"/> (depth-first). Returns null if not found.
    /// </summary>
    private static ElementDesc? FindDesc(LayoutDesc ld, uint id)
    {
        foreach (var kv in ld.Elements)
        {
            var f = FindDescIn(kv.Value, id);
            if (f is not null) return f;
        }
        return null;
    }

    private static ElementDesc? FindDescIn(ElementDesc d, uint id)
    {
        if (d.ElementId == id) return d;
        foreach (var kv in d.Children)
        {
            var f = FindDescIn(kv.Value, id);
            if (f is not null) return f;
        }
        return null;
    }

    // ── Sub-layout slot sizing helpers ────────────────────────────────────────

    /// <summary>
    /// Recursively propagates a parent-height change from <paramref name="oldParentH"/>
    /// to <paramref name="newParentH"/> through <paramref name="el"/> and all its
    /// descendants, replicating <c>UIElement::UpdateForParentSizeChange</c> from the
    /// retail runtime.
    ///
    /// <para>Three anchor cases for the vertical axis:
    /// <list type="bullet">
    ///   <item>Top+Bottom (stretch): maintain top margin AND bottom margin; height grows.</item>
    ///   <item>Bottom only (pin-to-bottom, fixed height): maintain bottom margin; Y moves, H unchanged.</item>
    ///   <item>Top only or None: Y and H unchanged (pinned to top with fixed height).</item>
    /// </list>
    /// Recurse with the element's new height so grandchildren see the correct parent size.</para>
    ///
    /// <para>This is called once at import time on base-children that are mounted into a
    /// slot taller than the sub-layout's design height.</para>
    /// </summary>
    private static void CascadeHeight(ElementInfo el, float oldParentH, float newParentH)
    {
        float oldH   = el.Height;
        float newH   = el.Height;
        float oldY   = el.Y;

        var anchors = ElementReader.ToAnchors(el.Left, el.Top, el.Right, el.Bottom);

        bool hasTop    = (anchors & AnchorEdges.Top)    != 0;
        bool hasBottom = (anchors & AnchorEdges.Bottom) != 0;

        if (hasTop && hasBottom)
        {
            // Stretch: maintain both top and bottom margins.
            // amT = el.Y (pinned to top, unchanged)
            // amB = oldParentH - (el.Y + el.H)
            float amB = oldParentH - (el.Y + el.Height);
            newH = newParentH - el.Y - amB;
            if (newH < 0f) newH = 0f;
            el.Height = newH;
        }
        else if (hasBottom && !hasTop)
        {
            // Pin to bottom: maintain bottom margin, height fixed, Y moves.
            // amB = oldParentH - (el.Y + el.H)
            float amB = oldParentH - (el.Y + el.Height);
            float newY = newParentH - amB - el.Height;
            el.Y = newY;
            // Height unchanged; newH = oldH for recursion.
        }
        // else: Top-only or None — element is top-pinned with fixed height; nothing moves.

        // Recurse into children with the element's new height as their parent.
        foreach (var child in el.Children)
            CascadeHeight(child, oldH, newH);
    }
}