using System.Linq;
using AcDream.App.UI;
using AcDream.App.UI.Layout;

namespace AcDream.App.Tests.UI.Layout;

/// <summary>
/// Dat-free conformance tests for the committed <c>options_2100002B.json</c> /
/// <c>options_gameplay_2100002A.json</c> / <c>options_character_21000028.json</c> /
/// <c>options_chat_2100005C.json</c> / <c>options_config_21000029.json</c> golden
/// fixtures (Campaign OP slice OP2, reworked 2026-08-11). Pins the tab table (dat
/// property <c>0x2E</c>), the three ListBox row-template lists (dat property
/// <c>0x64</c>), scrollbar linkage (dat property <c>0x72</c>), the new Type-8/5/
/// <c>0x10000036..0x44</c> widget mappings, and <see cref="UiTabPanel"/>'s switch
/// behavior. Retail anchors: <c>docs/research/2026-08-10-options-panel-structure.md</c>
/// §1.3, §1.5, §10.1. See also <c>OP2ReworkBlastRadiusConformanceTests.cs</c> (built-
/// widget pins for the pre-existing shipped panels the rework's dormancy model
/// protects) and the reader-level tests appended to <c>ElementReaderTests.cs</c>.
/// </summary>
public class OptionsPanelLayoutConformanceTests
{
    private static (uint, int, int) NoTex(uint _) => (0, 0, 0);

    private static ElementInfo? Find(ElementInfo n, uint id)
    {
        if (n.Id == id) return n;
        foreach (var c in n.Children)
        {
            var f = Find(c, id);
            if (f is not null) return f;
        }
        return null;
    }

    // ── Tab table (property 0x2E) ───────────────────────────────────────────

    [Fact]
    public void OptionsPanelFixture_TabControl_HasFourEntriesInAuthoredOrder_GameplayDefault()
    {
        var root = FixtureLoader.LoadOptionsPanelInfos();
        var tabControl = Find(root, 0x10000208u);
        Assert.NotNull(tabControl);
        Assert.Equal(8u, tabControl!.Type);

        var tabs = tabControl.TabTable;
        Assert.Equal(4, tabs.Count);

        Assert.Equal(new UiTabTableEntry(0x1000020Du, 0x10000212u, true), tabs[0]);  // Gameplay Options (default)
        Assert.Equal(new UiTabTableEntry(0x1000020Eu, 0x10000211u, false), tabs[1]); // Character
        Assert.Equal(new UiTabTableEntry(0x1000050Bu, 0x1000050Cu, false), tabs[2]); // Chat
        Assert.Equal(new UiTabTableEntry(0x1000020Fu, 0x10000213u, false), tabs[3]); // Config

        Assert.Single(tabs, t => t.IsDefault);
    }

    [Fact]
    public void OptionsPanelFixture_TabControl_BuildsAsUiTabPanel_WithSameTabTable()
    {
        var layout = FixtureLoader.LoadOptionsPanel();
        var tabControl = Assert.IsType<UiTabPanel>(layout.FindElement(0x10000208u));
        Assert.Equal(4, tabControl.Tabs.Count);
        Assert.True(tabControl.Tabs[0].IsDefault);
        Assert.Equal(0x10000212u, tabControl.Tabs[0].PageElementId);

        // OP2 rework: dormant until a controller opts in — importing alone must not
        // have switched anything (docs/research/2026-08-11-op2-review-blast.md).
        Assert.False(tabControl.BehaviorActive);
        Assert.Equal(0u, tabControl.ActivePageElementId);
    }

    // ── Row-template lists (property 0x64) ──────────────────────────────────

    [Fact]
    public void CharacterListBoxFixture_TemplateList_MatchesThreeRowTemplates()
    {
        var root = FixtureLoader.LoadOptionsCharacterInfos();
        var listBox = Find(root, 0x100001FAu);
        Assert.NotNull(listBox);
        Assert.Equal(5u, listBox!.Type);

        var templates = listBox.TemplateList;
        Assert.Equal(3, templates.Count);
        Assert.Equal(new UiTemplateListEntry(0x2100002Bu, 0x10000216u), templates[0]); // header
        Assert.Equal(new UiTemplateListEntry(0x2100002Bu, 0x10000217u), templates[1]); // separator
        Assert.Equal(new UiTemplateListEntry(0x2100002Bu, 0x10000218u), templates[2]); // toggle
    }

    [Fact]
    public void ConfigListBoxFixture_TemplateList_MatchesEightRowTemplates()
    {
        var root = FixtureLoader.LoadOptionsConfigInfos();
        var listBox = Find(root, 0x10000200u);
        Assert.NotNull(listBox);

        var templates = listBox!.TemplateList;
        Assert.Equal(8, templates.Count);
        uint[] expectedElementIds =
        {
            0x10000216u, 0x10000217u, 0x10000218u, 0x1000021Au,
            0x10000222u, 0x10000220u, 0x1000021Du, 0x10000221u,
        };
        for (int i = 0; i < expectedElementIds.Length; i++)
        {
            Assert.Equal(0x2100002Bu, templates[i].TemplateLayoutId);
            Assert.Equal(expectedElementIds[i], templates[i].TemplateElementId);
        }
    }

    [Fact]
    public void ChatListBoxFixture_TemplateList_MatchesNineRowTemplatesInclBitfield64()
    {
        var root = FixtureLoader.LoadOptionsChatInfos();
        var listBox = Find(root, 0x1000050Du);
        Assert.NotNull(listBox);

        var templates = listBox!.TemplateList;
        Assert.Equal(9, templates.Count);
        uint[] expectedElementIds =
        {
            0x10000216u, 0x10000217u, 0x10000218u, 0x1000021Au, 0x10000222u,
            0x10000220u, 0x1000021Du, 0x10000221u, 0x10000520u, // bitfield64
        };
        for (int i = 0; i < expectedElementIds.Length; i++)
        {
            Assert.Equal(0x2100002Bu, templates[i].TemplateLayoutId);
            Assert.Equal(expectedElementIds[i], templates[i].TemplateElementId);
        }
    }

    // ── Scrollbar linkage (property 0x72) ───────────────────────────────────

    [Fact]
    public void CharacterListBoxFixture_ScrollbarLinkage_Names0x100001FB()
    {
        var root = FixtureLoader.LoadOptionsCharacterInfos();
        var listBox = Find(root, 0x100001FAu);
        Assert.Equal(0x100001FBu, listBox!.ScrollbarElementId);
    }

    [Fact]
    public void ConfigListBoxFixture_ScrollbarLinkage_Names0x10000201()
    {
        var root = FixtureLoader.LoadOptionsConfigInfos();
        var listBox = Find(root, 0x10000200u);
        Assert.Equal(0x10000201u, listBox!.ScrollbarElementId);
    }

    [Fact]
    public void ChatListBoxFixture_ScrollbarLinkage_SharesElement0x10000201WithConfig()
    {
        // Research doc §10.1: Config's and Chat's ListBoxes both name the SAME
        // scrollbar element id — the two tabs share one gutter widget authored twice.
        var root = FixtureLoader.LoadOptionsChatInfos();
        var listBox = Find(root, 0x1000050Du);
        Assert.Equal(0x10000201u, listBox!.ScrollbarElementId);
    }

    // ── Built-widget mapping: ListBox → UiTemplateListBox ───────────────────

    [Fact]
    public void OptionsPanelFixture_CharacterListBox_BuildsAsUiTemplateListBoxWithSameData()
    {
        var layout = FixtureLoader.LoadOptionsPanel();
        var listBox = Assert.IsType<UiTemplateListBox>(layout.FindElement(0x100001FAu));
        Assert.Equal(3, listBox.Templates.Count);
        Assert.Equal(0x100001FBu, listBox.ScrollbarElementId);

        // OP2 rework: dormant until a controller adds its first row — importing alone
        // must not have injected a viewport child (docs/research/2026-08-11-op2-review-
        // blast.md SHOULD-FIX 1).
        Assert.Empty(listBox.Children);
        Assert.Equal(0, listBox.ContentHeight);
    }

    [Fact]
    public void OptionsPanelFixture_CharacterScrollbar_StillBuildsAsUiScrollbar()
    {
        // Regression guard: a real scrollbar sibling (Type 11) must keep building
        // through the EXISTING BuildScrollbar mapping — OP2 must not disturb it.
        var layout = FixtureLoader.LoadOptionsPanel();
        Assert.IsType<UiScrollbar>(layout.FindElement(0x100001FBu));
    }

    // ── The four remaining UIOption_* widget mappings ───────────────────────

    [Fact]
    public void OptionsPanelFixture_SliderControl_BuildsAsHorizontalUiScrollbar()
    {
        // UIOption_Slider (0x10000037), template element 0x1000021C: 120x12,
        // W>H so BuildScrollbar's existing horizontal branch applies, with the
        // child-id-1 thumb convention it already implements.
        var layout = FixtureLoader.LoadOptionsPanel();
        var slider = Assert.IsType<UiScrollbar>(layout.FindElement(0x1000021Cu));
        Assert.True(slider.Horizontal);
        Assert.NotEqual(0u, slider.ThumbSprite);
    }

    [Fact]
    public void OptionsPanelFixture_MenuControl_BuildsAsUiMenu()
    {
        // UIOption_Menu (0x10000038), template element 0x10000224.
        var layout = FixtureLoader.LoadOptionsPanel();
        Assert.IsType<UiMenu>(layout.FindElement(0x10000224u));
    }

    [Fact]
    public void OptionsPanelFixture_ToggleSliderRow_ComposesCheckboxAndSlider()
    {
        // UIOption_CheckboxSlider (0x10000036), unlabelled row template 0x10000220:
        // composes an existing UIOption_Checkbox (0x10000219 -> UiButton) and an
        // existing UIOption_Slider (0x1000021C -> UiScrollbar) — no new drawing code.
        // (0x10000219/0x1000021C are reused verbatim across every checkbox/slider
        // template instance, so this asserts identity against row.Children directly
        // rather than layout.FindElement — which, like CharacterStatController's own
        // documented caveat, returns only the LAST of several same-id duplicates.)
        var layout = FixtureLoader.LoadOptionsPanel();
        var row = Assert.IsType<UiOptionToggleSlider>(layout.FindElement(0x10000220u));
        Assert.NotNull(row.Toggle);
        Assert.NotNull(row.Slider);
        Assert.Contains(row.Toggle, row.Children);
        Assert.Contains(row.Slider, row.Children);
        Assert.Equal(0x10000219u, row.Toggle!.DatElementId);
        Assert.Equal(0x1000021Cu, row.Slider!.DatElementId);
    }

    [Fact]
    public void OptionsPanelFixture_LabelledToggleSliderRow_AlsoComposesCheckboxAndSlider()
    {
        // Labelled variant 0x10000221 (four children: checkbox + slider + two range
        // labels) — the two extra Text children must not confuse the composition.
        var layout = FixtureLoader.LoadOptionsPanel();
        var row = Assert.IsType<UiOptionToggleSlider>(layout.FindElement(0x10000221u));
        Assert.NotNull(row.Toggle);
        Assert.NotNull(row.Slider);
    }

    [Fact]
    public void OptionsPanelFixture_Bitfield64Template_BuildsAsEmptyUiCheckboxBitfield64()
    {
        // UIOption_CheckboxBitfield64 (0x10000044), template element 0x10000520:
        // authors zero children/media of its OWN, but DOES author its own row-template
        // list (dat property 0x64 -> {0x2100002B, 0x10000521}) — every row is built
        // from THAT template at runtime (OP2 rework,
        // docs/research/2026-08-11-op2-review-mechanism.md MUST-FIX 4).
        var layout = FixtureLoader.LoadOptionsPanel();
        var bitfield = Assert.IsType<UiCheckboxBitfield64>(layout.FindElement(0x10000520u));
        Assert.Empty(bitfield.Rows);
        UiTemplateListEntry template = Assert.Single(bitfield.Templates);
        Assert.Equal(new UiTemplateListEntry(0x2100002Bu, 0x10000521u), template);
    }

    [Fact]
    public void OptionsPanelFixture_LabelledSliderRow_StillPlainContainer()
    {
        // Template idx 6 (0x1000021D) is the LABELLED slider (label + slider + two
        // range labels) but its OWN root Type is 3 (plain container), not
        // UIOption_CheckboxSlider — distinguishing this from 0x10000221 is exactly
        // what makes the mapping data-driven rather than guessed.
        var layout = FixtureLoader.LoadOptionsPanel();
        var row = layout.FindElement(0x1000021Du);
        Assert.IsNotType<UiOptionToggleSlider>(row);
        var slider = Assert.IsType<UiScrollbar>(layout.FindElement(0x1000021Cu));
        Assert.NotNull(slider);
    }

    // ── UiCheckboxBitfield64 behavior (rows built from the authored template) ──

    /// <summary>Builds a hermetic <see cref="UiCheckboxBitfield64"/> whose
    /// <see cref="UiCheckboxBitfield64.TemplateResolver"/> constructs a minimal subtree
    /// containing ONLY the checkbox retail's own CreateChildren looks for
    /// (<see cref="UiCheckboxBitfield64.TemplateCheckboxElementId"/>) — proves the
    /// widget resolves and stamps that exact id rather than assuming row shape.</summary>
    private static UiCheckboxBitfield64 NewBitfieldWithStubTemplate(
        uint checkedLedSprite = 0u, uint uncheckedLedSprite = 0u)
    {
        var templates = new[] { new UiTemplateListEntry(0x2100002Bu, 0x10000521u) };
        return new UiCheckboxBitfield64(templates, checkedLedSprite, uncheckedLedSprite)
        {
            Width = 272f,
            TemplateResolver = (layoutId, elementId) =>
            {
                Assert.Equal(0x2100002Bu, layoutId);
                Assert.Equal(0x10000521u, elementId);
                var checkboxInfo = new ElementInfo
                {
                    Id = UiCheckboxBitfield64.TemplateCheckboxElementId,
                    Type = 1u,
                    Width = 260f,
                    Height = 14f,
                };
                return LayoutImporter.Build(checkboxInfo, NoTex, null).Root;
            },
        };
    }

    [Fact]
    public void UiCheckboxBitfield64_AddChild_ResolvesRowFromAuthoredTemplate()
    {
        var bitfield = NewBitfieldWithStubTemplate();
        bitfield.SetDefaultValue(0x00000000_FBFFFFFFul, 0ul); // retail main-window default (research §5.2)

        // "Error" (mask 0x04000000) is the ONE bit clear in the FBFFFFFF default —
        // research §5.2's own cross-check ("Society bit opt-in") confirms every
        // other row's bits are already set in that default, so Error is the only
        // mask that starts observably OFF and round-trips cleanly through OR/AND-NOT.
        UiButton? row = bitfield.AddChild(0x00000000_04000000ul, 0ul, "Error");
        Assert.NotNull(row);
        Assert.Equal(UiCheckboxBitfield64.TemplateCheckboxElementId, row!.DatElementId);
        Assert.False(row.Selected);

        ulong? changedLow = null;
        bitfield.ValueChanged += (low, _) => changedLow = low;
        row.OnClick!.Invoke();

        Assert.True(row.Selected);
        Assert.Equal(0x00000000_FFFFFFFFul, changedLow); // FBFFFFFF | 04000000 == FFFFFFFF

        row.OnClick!.Invoke();
        Assert.False(row.Selected);
        Assert.Equal(0x00000000_FBFFFFFFul, changedLow); // back to the seeded default
    }

    /// <summary>
    /// MUST-FIX 3a (`docs/research/2026-08-11-op2-review-mechanism.md`): retail
    /// <c>UIOption_CheckboxBitfield64::Refresh @0x004859C0</c> checks a row when ANY
    /// mask bit is set, not when EVERY mask bit is set. A single-bit mask cannot
    /// distinguish the two predicates — this seeds a value with only ONE of a
    /// TWO-bit mask's bits set (Combat's real mask <c>0x00600040</c>, research §5.2)
    /// and asserts the row still reads checked.
    /// </summary>
    [Fact]
    public void UiCheckboxBitfield64_MultiBitMask_IsSet_UsesAnyBitNotAllBits()
    {
        var bitfield = NewBitfieldWithStubTemplate();
        const ulong combatMask = 0x00000000_00600040ul; // two bits
        const ulong onlyOneBitOfMask = 0x00000000_00000040ul; // partially overlapping
        bitfield.SetDefaultValue(onlyOneBitOfMask, 0ul);

        UiButton? row = bitfield.AddChild(combatMask, 0ul, "Combat");
        Assert.NotNull(row);
        Assert.True(row!.Selected); // any-bit predicate: 0x40 & 0x600040 != 0

        // Clicking now must turn the row OFF (it currently reads checked) and clear
        // BOTH mask bits, matching retail's confirmed SetBitsOnOrOff AND-NOT.
        ulong? changedLow = null;
        bitfield.ValueChanged += (low, _) => changedLow = low;
        row.OnClick!.Invoke();
        Assert.False(row.Selected);
        Assert.Equal(0ul, changedLow);
    }

    [Fact]
    public void UiCheckboxBitfield64_AddChild_NoTemplateResolverWired_ReturnsNullAndLeavesNoRow()
    {
        var templates = new[] { new UiTemplateListEntry(0x2100002Bu, 0x10000521u) };
        var bitfield = new UiCheckboxBitfield64(templates) { Width = 272f };
        // No TemplateResolver wired — matches DatWidgetFactory's own construction
        // (OP2 ships the mechanism; a page controller supplies the resolver).
        Assert.Null(bitfield.AddChild(0x1ul, 0ul, "Unwired"));
        Assert.Empty(bitfield.Rows);
    }

    // ── AP-195: LED media swap + self-sizing (Campaign OP slice OP5) ────────

    private const uint CheckedLedDid = 0x06004D17u;
    private const uint UncheckedLedDid = 0x06004D19u;

    [Fact]
    public void AddChild_AllMaskBitsSet_AppliesCheckedLedSprite()
    {
        var bitfield = NewBitfieldWithStubTemplate(CheckedLedDid, UncheckedLedDid);
        const ulong combatMask = 0x00000000_00600040ul; // two bits
        bitfield.SetDefaultValue(combatMask, 0ul); // BOTH bits already set

        UiButton? row = bitfield.AddChild(combatMask, 0ul, "Combat");

        Assert.NotNull(row);
        Assert.True(row!.Selected);
        Assert.Equal(CheckedLedDid, row.FaceFileOverride);
    }

    [Fact]
    public void AddChild_PartialMaskBitsSet_AppliesUncheckedLedSprite()
    {
        var bitfield = NewBitfieldWithStubTemplate(CheckedLedDid, UncheckedLedDid);
        const ulong combatMask = 0x00000000_00600040ul; // two bits
        const ulong onlyOneBitOfMask = 0x00000000_00000040ul;
        bitfield.SetDefaultValue(onlyOneBitOfMask, 0ul); // checked (any-set) but not all-set

        UiButton? row = bitfield.AddChild(combatMask, 0ul, "Combat");

        Assert.NotNull(row);
        Assert.True(row!.Selected); // any-set half unaffected by AP-195
        Assert.Equal(UncheckedLedDid, row.FaceFileOverride);
    }

    [Fact]
    public void AddChild_NoMaskBitsSet_LeavesLedOverrideUnset()
    {
        var bitfield = NewBitfieldWithStubTemplate(CheckedLedDid, UncheckedLedDid);
        bitfield.SetDefaultValue(0ul, 0ul);

        UiButton? row = bitfield.AddChild(0x00000000_00600040ul, 0ul, "Combat");

        Assert.NotNull(row);
        Assert.False(row!.Selected);
        Assert.Null(row.FaceFileOverride); // retail's own gate: swap only applies while checked
    }

    [Fact]
    public void ToggleRow_TogglingOff_ClearsLedOverride()
    {
        var bitfield = NewBitfieldWithStubTemplate(CheckedLedDid, UncheckedLedDid);
        const ulong singleBitMask = 0x00000000_00000040ul;
        bitfield.SetDefaultValue(singleBitMask, 0ul); // fully set -> checked LED

        UiButton row = bitfield.AddChild(singleBitMask, 0ul, "Combat")!;
        Assert.Equal(CheckedLedDid, row.FaceFileOverride);

        row.OnClick!.Invoke(); // turns OFF (was fully set)

        Assert.False(row.Selected);
        Assert.Null(row.FaceFileOverride);
    }

    [Fact]
    public void AddChild_MissingLedSprites_LeavesOverrideNull_EvenWhileChecked()
    {
        // Constructed WITHOUT checkedLedSprite/uncheckedLedSprite (0/0 default) —
        // e.g. an authoring gap or a fixture that predates AP-195's properties.
        var bitfield = NewBitfieldWithStubTemplate();
        const ulong mask = 0x1ul;
        bitfield.SetDefaultValue(mask, 0ul);

        UiButton? row = bitfield.AddChild(mask, 0ul, "Row");

        Assert.NotNull(row);
        Assert.True(row!.Selected);
        Assert.Null(row.FaceFileOverride);
    }

    [Fact]
    public void AddChild_MultipleRows_SelfSizesHeightToStackedContent()
    {
        // AP-195's self-sizing tail: retail's ResizeTo(GetWidth(), CalculatePaperSize)
        // — the block grows to fit N rows instead of keeping its authored 272x100
        // extent. Each stub row template is 14px tall (NewBitfieldWithStubTemplate).
        var bitfield = NewBitfieldWithStubTemplate();
        bitfield.SetDefaultValue(0ul, 0ul);
        Assert.Equal(0f, bitfield.Height);

        bitfield.AddChild(0x1ul, 0ul, "Row1");
        Assert.Equal(14f, bitfield.Height);

        bitfield.AddChild(0x2ul, 0ul, "Row2");
        bitfield.AddChild(0x4ul, 0ul, "Row3");

        Assert.Equal(42f, bitfield.Height); // 3 * 14
        Assert.Equal(272f, bitfield.Width); // Width (GetWidth()) is untouched
    }

    [Fact]
    public void SetCurrentValue_PushesValueAndVisuals_WithoutFiringValueChanged()
    {
        var bitfield = NewBitfieldWithStubTemplate(CheckedLedDid, UncheckedLedDid);
        const ulong mask = 0x1ul;
        bitfield.SetDefaultValue(0ul, 0ul); // starts unset
        UiButton row = bitfield.AddChild(mask, 0ul, "Row")!;
        Assert.False(row.Selected);

        bool valueChangedFired = false;
        bitfield.ValueChanged += (_, _) => valueChangedFired = true;

        bitfield.SetCurrentValue(mask, 0ul); // the OnShown/Reset/Defaults re-seed path

        Assert.Equal(mask, bitfield.CurrentLow);
        Assert.True(row.Selected);
        Assert.Equal(CheckedLedDid, row.FaceFileOverride);
        Assert.False(valueChangedFired); // a re-seed must never loop back as "user edited this"
    }

    // ── UiTemplateListBox.AddPrebuiltRow (AP-195's "reuse the ListBox's own
    //    stacking" disposition) ──────────────────────────────────────────────

    [Fact]
    public void AddPrebuiltRow_StacksBelowPreviousContent_UsingTheRowsFinalHeight()
    {
        ElementInfo panelRoot = FixtureLoader.LoadOptionsPanelInfos();
        ElementInfo characterListBoxInfo = Find(FixtureLoader.LoadOptionsCharacterInfos(), 0x100001FAu)!;
        var listBox = new UiTemplateListBox(
            characterListBoxInfo, NoTex,
            characterListBoxInfo.TemplateList, characterListBoxInfo.ScrollbarElementId)
        {
            TemplateResolver = (layoutId, elementId) =>
            {
                if (layoutId != 0x2100002Bu) return null;
                ElementInfo? templateInfo = Find(panelRoot, elementId);
                return templateInfo is null ? null : LayoutImporter.Build(templateInfo, NoTex, null).Root;
            },
        };

        // A first ordinary row via the normal template path (header, 22px).
        UiElement? header = listBox.AddItemFromTemplateList(0);
        Assert.NotNull(header);
        Assert.Equal(22, listBox.ContentHeight);

        // A pre-sized row built OUTSIDE the ListBox's own resolve step (the shape a
        // self-sizing UiCheckboxBitfield64 block uses) — its height is whatever the
        // caller already grew it to BEFORE calling AddPrebuiltRow.
        var prebuilt = new UiPanel { Width = 272f, Height = 260f };

        UiElement returned = listBox.AddPrebuiltRow(prebuilt);

        Assert.Same(prebuilt, returned);
        Assert.Equal(22f, prebuilt.Top); // stacked directly below the header row
        Assert.Equal(22 + 260, listBox.ContentHeight); // the FINAL 260px height, not a stale one
        Assert.Contains(prebuilt, listBox.Children.OfType<UiScrollablePanel>().Single().Children);
    }

    [Fact]
    public void AddPrebuiltRow_SubsequentTemplateRow_StacksAfterThePrebuiltRowsFinalHeight()
    {
        ElementInfo characterListBoxInfo = Find(FixtureLoader.LoadOptionsCharacterInfos(), 0x100001FAu)!;
        var listBox = new UiTemplateListBox(
            characterListBoxInfo, NoTex,
            characterListBoxInfo.TemplateList, characterListBoxInfo.ScrollbarElementId)
        {
            TemplateResolver = (layoutId, elementId) =>
            {
                if (layoutId != 0x2100002Bu) return null;
                ElementInfo? templateInfo = Find(FixtureLoader.LoadOptionsPanelInfos(), elementId);
                return templateInfo is null ? null : LayoutImporter.Build(templateInfo, NoTex, null).Root;
            },
        };

        var prebuilt = new UiPanel { Width = 272f, Height = 260f };
        listBox.AddPrebuiltRow(prebuilt);
        Assert.Equal(260, listBox.ContentHeight);

        // A separator row added AFTER the prebuilt block must stack below its FINAL
        // 260px height, not the block's pre-build authored 100px extent — this is
        // exactly the reflow problem AddPrebuiltRow exists to avoid.
        UiElement? separator = listBox.AddItemFromTemplateList(1); // separator template
        Assert.NotNull(separator);
        Assert.Equal(260f, separator!.Top);
    }

    // ── The template mechanism, exercised end-to-end ────────────────────────

    /// <summary>
    /// Proves the Character ListBox's authored structure (6 headers + 49 toggles,
    /// research doc §7 / §10.1) is REACHABLE through
    /// <see cref="UiTemplateListBox.AddItemFromTemplateList"/> — each call resolves
    /// its template through the SAME <c>LayoutImporter.Build</c> machinery a real
    /// controller would use, sourced from the committed <c>options_2100002B.json</c>
    /// fixture (the templates' one true home).
    /// </summary>
    [Fact]
    public void CharacterListBox_TemplateMechanism_ReachesSixHeadersAndFortyNineToggles()
    {
        ElementInfo panelRoot = FixtureLoader.LoadOptionsPanelInfos();
        UiElement? Resolve(uint layoutId, uint elementId)
        {
            if (layoutId != 0x2100002Bu) return null;
            ElementInfo? templateInfo = Find(panelRoot, elementId);
            return templateInfo is null ? null : LayoutImporter.Build(templateInfo, NoTex, null).Root;
        }

        ElementInfo characterListBoxInfo = Find(FixtureLoader.LoadOptionsCharacterInfos(), 0x100001FAu)!;
        var listBox = new UiTemplateListBox(
            characterListBoxInfo, NoTex,
            characterListBoxInfo.TemplateList, characterListBoxInfo.ScrollbarElementId)
        {
            TemplateResolver = Resolve,
        };

        // Lane B §7 authored section row counts (docs/research/2026-08-10-options-panel-structure.md):
        // User Interface Behavior=3, User Interface Display=15, Grouping=6,
        // Other Players=11, Character Behavior=7, Chat=7 -> 49 toggles under 6 headers.
        int[] sectionToggleCounts = { 3, 15, 6, 11, 7, 7 };
        int headerCalls = 0, toggleCalls = 0;
        foreach (int toggleCount in sectionToggleCounts)
        {
            var header = listBox.AddItemFromTemplateList(0); // header template
            Assert.IsType<UiText>(header);
            headerCalls++;

            for (int i = 0; i < toggleCount; i++)
            {
                var toggleRow = listBox.AddItemFromTemplateList(2); // toggle template
                Assert.NotNull(toggleRow);
                // Toggle template 0x10000218 is a plain Type-3 container wrapping ONE
                // UIOption_Checkbox child — proves the SAME BuildCheckbox mapping
                // fires through the template mechanism, not just for authored trees.
                Assert.IsType<UiButton>(Assert.Single(toggleRow!.Children));
                toggleCalls++;
            }
        }

        Assert.Equal(6, headerCalls);
        Assert.Equal(49, toggleCalls);

        // 6 headers @22px + 49 toggles @20px (template heights 0x10000216/0x10000218).
        Assert.Equal(6 * 22 + 49 * 20, listBox.ContentHeight);
    }

    [Fact]
    public void TemplateListBox_AddItemFromTemplateList_WithoutResolver_ReturnsNull()
    {
        ElementInfo characterListBoxInfo = Find(FixtureLoader.LoadOptionsCharacterInfos(), 0x100001FAu)!;
        var listBox = new UiTemplateListBox(
            characterListBoxInfo, NoTex,
            characterListBoxInfo.TemplateList, characterListBoxInfo.ScrollbarElementId);
        // No TemplateResolver wired — matches DatWidgetFactory's own construction
        // (OP2 ships the mechanism; a page controller supplies the resolver).
        Assert.Null(listBox.AddItemFromTemplateList(0));
        // OP2 rework: a failed/never-attempted add must not have allocated a viewport.
        Assert.Empty(listBox.Children);
    }

    [Fact]
    public void TemplateListBox_AddItemFromTemplateList_OutOfRangeIndex_ReturnsNull()
    {
        var listBox = new UiTemplateListBox(
            new ElementInfo(), NoTex, new[] { new UiTemplateListEntry(1u, 2u) }, 0u)
        {
            TemplateResolver = (_, _) => new UiDatElement(new ElementInfo(), NoTex),
        };
        Assert.Null(listBox.AddItemFromTemplateList(-1));
        Assert.Null(listBox.AddItemFromTemplateList(1));
        Assert.NotNull(listBox.AddItemFromTemplateList(0));
        // The one successful add DID lazily create the viewport.
        Assert.Single(listBox.Children);
    }

    // ── UiTabPanel behavior ──────────────────────────────────────────────────
    // OP2 rework: these all explicitly call ActivateTabBehavior() first — a Type-8
    // element is dormant on import (docs/research/2026-08-11-op2-review-blast.md /
    // -mechanism.md), so a controller (here, the test itself) must opt in before any
    // switching happens. This mirrors how OP3+'s OptionsPanelController will drive it.

    [Fact]
    public void UiTabPanel_ActivateTabBehavior_ShowsOnlyTheGameplayDefaultPage()
    {
        var layout = FixtureLoader.LoadOptionsPanel();
        var tabControl = Assert.IsType<UiTabPanel>(layout.FindElement(0x10000208u));

        tabControl.ActivateTabBehavior();

        Assert.True(tabControl.BehaviorActive);
        Assert.Empty(tabControl.UnresolvedEntries);
        Assert.Equal(0x10000212u, tabControl.ActivePageElementId); // Gameplay page slot

        UiElement gameplayPage = layout.FindElement(0x10000212u)!;
        UiElement characterPage = layout.FindElement(0x10000211u)!;
        UiElement chatPage = layout.FindElement(0x1000050Cu)!;
        UiElement configPage = layout.FindElement(0x10000213u)!;

        Assert.True(gameplayPage.Visible);
        Assert.False(characterPage.Visible);
        Assert.False(chatPage.Visible);
        Assert.False(configPage.Visible);
    }

    [Fact]
    public void UiTabPanel_ActivateTabBehavior_IsIdempotent()
    {
        var layout = FixtureLoader.LoadOptionsPanel();
        var tabControl = Assert.IsType<UiTabPanel>(layout.FindElement(0x10000208u));

        tabControl.ActivateTabBehavior();
        tabControl.SwitchTo(0x10000211u); // Character page slot — deliberately not the default
        tabControl.ActivateTabBehavior(); // second call must be a no-op, not re-switch to default

        Assert.Equal(0x10000211u, tabControl.ActivePageElementId);
    }

    [Fact]
    public void UiTabPanel_SwitchTo_ShowsExactlyOnePage()
    {
        var layout = FixtureLoader.LoadOptionsPanel();
        var tabControl = Assert.IsType<UiTabPanel>(layout.FindElement(0x10000208u));
        tabControl.ActivateTabBehavior();

        tabControl.SwitchTo(0x1000050Cu); // Chat page slot

        UiElement gameplayPage = layout.FindElement(0x10000212u)!;
        UiElement characterPage = layout.FindElement(0x10000211u)!;
        UiElement chatPage = layout.FindElement(0x1000050Cu)!;
        UiElement configPage = layout.FindElement(0x10000213u)!;

        var visiblePages = new[] { gameplayPage, characterPage, chatPage, configPage }
            .Where(p => p.Visible)
            .ToList();
        Assert.Single(visiblePages);
        Assert.Same(chatPage, visiblePages[0]);
        Assert.Equal(0x1000050Cu, tabControl.ActivePageElementId);
    }

    [Fact]
    public void UiTabPanel_SwitchTo_UpdatesTabButtonOpenClosedState()
    {
        var layout = FixtureLoader.LoadOptionsPanel();
        var tabControl = Assert.IsType<UiTabPanel>(layout.FindElement(0x10000208u));
        tabControl.ActivateTabBehavior();

        var gameplayButton = Assert.IsType<UiText>(layout.FindElement(0x1000020Du));
        var characterButton = Assert.IsType<UiText>(layout.FindElement(0x1000020Eu));

        Assert.Equal(RetailUiStateIds.Open, gameplayButton.ActiveRetailStateId);
        Assert.Equal(RetailUiStateIds.Closed, characterButton.ActiveRetailStateId);

        tabControl.SwitchTo(0x10000211u); // Character page slot

        Assert.Equal(RetailUiStateIds.Closed, gameplayButton.ActiveRetailStateId);
        Assert.Equal(RetailUiStateIds.Open, characterButton.ActiveRetailStateId);
    }

    [Fact]
    public void UiTabPanel_ClickingTabButton_SwitchesActivePage()
    {
        var layout = FixtureLoader.LoadOptionsPanel();
        var tabControl = Assert.IsType<UiTabPanel>(layout.FindElement(0x10000208u));
        tabControl.ActivateTabBehavior(); // clicks only bind once behavior is activated
        var characterButton = layout.FindElement(0x1000020Eu)!;

        characterButton.OnEvent(new UiEvent(0, characterButton, UiEventType.Click));

        Assert.Equal(0x10000211u, tabControl.ActivePageElementId);
        Assert.True(layout.FindElement(0x10000211u)!.Visible);
        Assert.False(layout.FindElement(0x10000212u)!.Visible);
    }

    [Fact]
    public void UiTabPanel_ClickingTabButton_BeforeActivation_DoesNothing()
    {
        // OP2 rework's central regression guard: a dormant instance's tab buttons are
        // NOT click-bound yet, so a click before ActivateTabBehavior() must be inert —
        // this is exactly the property that keeps the four pre-existing shipped Type-8
        // hosts (which never call ActivateTabBehavior) safe.
        var layout = FixtureLoader.LoadOptionsPanel();
        var tabControl = Assert.IsType<UiTabPanel>(layout.FindElement(0x10000208u));
        var characterButton = layout.FindElement(0x1000020Eu)!;

        characterButton.OnEvent(new UiEvent(0, characterButton, UiEventType.Click));

        Assert.Equal(0u, tabControl.ActivePageElementId);
        Assert.False(tabControl.BehaviorActive);
    }

    [Fact]
    public void UiTabPanel_SwitchToSameActivePage_IsANoOp()
    {
        var layout = FixtureLoader.LoadOptionsPanel();
        var tabControl = Assert.IsType<UiTabPanel>(layout.FindElement(0x10000208u));
        tabControl.ActivateTabBehavior();
        uint before = tabControl.ActivePageElementId;

        tabControl.SwitchTo(before);

        Assert.Equal(before, tabControl.ActivePageElementId);
        Assert.True(layout.FindElement(before)!.Visible);
    }

    // ── ActivePageChanged (Campaign OP slice OP3 — the page-model hook) ──────

    [Fact]
    public void ActivePageChanged_FiresOnInitialActivation_WithOldEqualsZero()
    {
        var layout = FixtureLoader.LoadOptionsPanel();
        var tabControl = Assert.IsType<UiTabPanel>(layout.FindElement(0x10000208u));
        var transitions = new List<(uint Old, uint New)>();
        tabControl.ActivePageChanged += (oldId, newId) => transitions.Add((oldId, newId));

        tabControl.ActivateTabBehavior();

        Assert.Equal([(0u, 0x10000212u)], transitions); // Gameplay default, no prior page.
    }

    [Fact]
    public void ActivePageChanged_FiresOnEverySwitch_WithBothOldAndNew()
    {
        var layout = FixtureLoader.LoadOptionsPanel();
        var tabControl = Assert.IsType<UiTabPanel>(layout.FindElement(0x10000208u));
        tabControl.ActivateTabBehavior();
        var transitions = new List<(uint Old, uint New)>();
        tabControl.ActivePageChanged += (oldId, newId) => transitions.Add((oldId, newId));

        tabControl.SwitchTo(0x1000050Cu); // Chat page slot

        Assert.Equal([(0x10000212u, 0x1000050Cu)], transitions);
    }

    [Fact]
    public void ActivePageChanged_DoesNotFireForANoOpSwitch()
    {
        var layout = FixtureLoader.LoadOptionsPanel();
        var tabControl = Assert.IsType<UiTabPanel>(layout.FindElement(0x10000208u));
        tabControl.ActivateTabBehavior();
        uint before = tabControl.ActivePageElementId;
        var transitions = new List<(uint Old, uint New)>();
        tabControl.ActivePageChanged += (oldId, newId) => transitions.Add((oldId, newId));

        tabControl.SwitchTo(before);

        Assert.Empty(transitions);
    }
}