feat(ui): port favorite spell bar overflow

Import the retail arrow-only spell bar scrollbar from LayoutDesc, preserve authored end-button extents and HideDisabled behavior in the shared retained widget, and bind each favorite list to its sole horizontal pixel-scroll model. Match retail selection exposure for off-screen spells and pin the behavior with real-fixture conformance tests.

Document the six-slice world-interaction completion program as the active pre-M4 work order.

Co-authored-by: Codex <codex@openai.com>

tests/AcDream.App.Tests/UI/Layout/DatWidgetFactoryTests.cs

@@ -223,6 +223,60 @@ public class DatWidgetFactoryTests
         Assert.Equal(0u, bar.DownSprite);
     }
 
+    [Fact]
+    public void Type11_HorizontalScrollbar_ImportsAuthoredArrowButtonsAndHideDisabled()
+    {
+        const uint DecrementId = 0x10000071u;
+        const uint IncrementId = 0x10000072u;
+        const uint DecrementSprite = 0x06004CDCu;
+        const uint IncrementSprite = 0x06004CDEu;
+        var decrement = new ElementInfo
+        {
+            Id = DecrementId, Type = 1u, Width = 23f, Height = 36f,
+        };
+        decrement.States[UiStateInfo.DirectStateId] = new UiStateInfo
+        {
+            Id = UiStateInfo.DirectStateId,
+            Image = new UiImageMedia(DecrementSprite, 1),
+        };
+        var increment = new ElementInfo
+        {
+            Id = IncrementId, Type = 1u, Width = 23f, Height = 36f,
+        };
+        increment.States[UiStateInfo.DirectStateId] = new UiStateInfo
+        {
+            Id = UiStateInfo.DirectStateId,
+            Image = new UiImageMedia(IncrementSprite, 1),
+        };
+        var info = new ElementInfo
+        {
+            Type = 11u,
+            Id = SpellcastingUiController.FavoriteScrollbarId,
+            Width = 685f,
+            Height = 36f,
+            Children = [decrement, increment],
+        };
+        var state = new UiStateInfo { Id = UiStateInfo.DirectStateId };
+        state.Properties.Values[0x77u] = new UiPropertyValue
+            { Kind = UiPropertyKind.Enum, UnsignedValue = IncrementId };
+        state.Properties.Values[0x78u] = new UiPropertyValue
+            { Kind = UiPropertyKind.Enum, UnsignedValue = DecrementId };
+        state.Properties.Values[0x79u] = Bool(true);
+        info.States[UiStateInfo.DirectStateId] = state;
+
+        var bar = Assert.IsType<UiScrollbar>(
+            DatWidgetFactory.Create(info, NoTex, null));
+
+        Assert.True(bar.Horizontal);
+        Assert.True(bar.HideWhenDisabled);
+        Assert.Equal(23f, bar.DecrementButtonExtent);
+        Assert.Equal(23f, bar.IncrementButtonExtent);
+        Assert.Equal(DecrementSprite, bar.UpSprite);
+        Assert.Equal(IncrementSprite, bar.DownSprite);
+        Assert.Equal(0u, bar.TrackSprite);
+        Assert.Equal(0u, bar.ThumbSprite);
+    }
+
     [Fact]
     public void RetailToolbarFixture_buildsEditableStackEntry_andAuthoredHorizontalSlider()
     {

tests/AcDream.App.Tests/UI/Layout/SpellcastingUiControllerTests.cs

@@ -34,8 +34,19 @@ public sealed class SpellcastingUiControllerTests
 
         UiElement group = layout.FindElement(0x100000AAu)!;
         UiItemList list = Descendants(group).OfType<UiItemList>().First();
+        UiScrollbar scrollbar = Descendants(group).OfType<UiScrollbar>().Single(
+            candidate => candidate.DatElementId == SpellcastingUiController.FavoriteScrollbarId);
         Assert.True(list.SingleRow);
+        Assert.True(list.HorizontalScroll);
         Assert.True(list.FillVisibleEmptySlots);
+        Assert.Same(list.Scroll, scrollbar.Model);
+        Assert.True(scrollbar.Horizontal);
+        Assert.True(scrollbar.HideWhenDisabled);
+        Assert.Equal(23f, scrollbar.DecrementButtonExtent);
+        Assert.Equal(23f, scrollbar.IncrementButtonExtent);
+        Assert.Equal(0x06004CDCu, scrollbar.UpSprite);
+        Assert.Equal(0x06004CDEu, scrollbar.DownSprite);
+        Assert.False(scrollbar.IsPresentationVisible);
         Assert.Equal(439f, list.Width);
         Assert.Equal(13, list.GetNumUIItems());
 
@@ -71,6 +82,43 @@ public sealed class SpellcastingUiControllerTests
             cast.FaceSegmentRectsForTest());
     }
 
+    [Fact]
+    public void FavoriteOverflow_ArrowButtonsStepOneCell_AndSelectionScrollsIntoView()
+    {
+        ImportedLayout layout = LayoutImporter.Build(
+            FixtureLoader.LoadCombatInfos(), NoTex, datFont: null);
+        ApplyAnchors(layout.Root);
+        var spellbook = new Spellbook();
+        for (uint spellId = 1u; spellId <= 20u; spellId++)
+        {
+            spellbook.OnSpellLearned(spellId, 1f);
+            spellbook.SetFavorite(0, (int)spellId - 1, spellId);
+        }
+        var objects = new ClientObjectTable();
+        objects.AddOrUpdate(new ClientObject { ObjectId = 1u, Name = "Player" });
+
+        using SpellcastingUiController controller = Bind(
+            layout, spellbook, objects, _ => { })!;
+
+        UiElement group = layout.FindElement(0x100000AAu)!;
+        UiItemList list = Descendants(group).OfType<UiItemList>().Single();
+        UiScrollbar scrollbar = Descendants(group).OfType<UiScrollbar>().Single(
+            candidate => candidate.DatElementId == SpellcastingUiController.FavoriteScrollbarId);
+        Assert.True(scrollbar.IsPresentationVisible);
+        Assert.Equal(201, list.Scroll.MaxScroll);
+
+        Assert.True(scrollbar.OnEvent(new UiEvent(
+            0u, scrollbar, UiEventType.MouseDown, Data1: (int)scrollbar.Width - 1)));
+        Assert.Equal(32, list.Scroll.ScrollY);
+        Assert.True(scrollbar.OnEvent(new UiEvent(
+            0u, scrollbar, UiEventType.MouseDown, Data1: 0)));
+        Assert.Equal(0, list.Scroll.ScrollY);
+
+        controller.AddFavorite(20u);
+
+        Assert.Equal(list.Scroll.MaxScroll, list.Scroll.ScrollY);
+    }
+
     [Fact]
     public void ImportedFixture_BindsAllTabs_AndTracksEquippedEndowment()
     {

tests/AcDream.App.Tests/UI/UiScrollbarTests.cs

@@ -128,6 +128,60 @@ public class UiScrollbarTests
         Assert.True(bar.OnEvent(new UiEvent(0u, bar, UiEventType.MouseUp, Data1: 144)));
     }
 
+    [Fact]
+    public void HorizontalModel_UsesAuthoredArrowExtentsAndOneCellStep()
+    {
+        var model = new UiScrollable
+        {
+            ContentHeight = 640,
+            ViewHeight = 320,
+            LineHeight = 32,
+        };
+        model.SetScrollY(64);
+        var bar = new UiScrollbar
+        {
+            Width = 160f,
+            Height = 36f,
+            Horizontal = true,
+            Model = model,
+            DecrementButtonExtent = 23f,
+            IncrementButtonExtent = 29f,
+        };
+
+        Assert.True(bar.OnEvent(new UiEvent(
+            0u, bar, UiEventType.MouseDown, Data1: 22)));
+        Assert.Equal(32, model.ScrollY);
+
+        Assert.True(bar.OnEvent(new UiEvent(
+            0u, bar, UiEventType.MouseDown, Data1: 131)));
+        Assert.Equal(64, model.ScrollY);
+    }
+
+    [Fact]
+    public void ModelWithoutOverflow_IsDisabledAndHideDisabledSuppressesPresentation()
+    {
+        var model = new UiScrollable
+        {
+            ContentHeight = 320,
+            ViewHeight = 320,
+            LineHeight = 32,
+        };
+        var bar = new UiScrollbar
+        {
+            Width = 160f,
+            Height = 36f,
+            Horizontal = true,
+            Model = model,
+            HideWhenDisabled = true,
+        };
+
+        Assert.True(bar.IsModelDisabled);
+        Assert.False(bar.IsPresentationVisible);
+        Assert.False(bar.OnEvent(new UiEvent(
+            0u, bar, UiEventType.MouseDown, Data1: 159)));
+        Assert.Equal(0, model.ScrollY);
+    }
+
     [Theory]
     [InlineData(0f,   0f,   0f)]
     [InlineData(0.5f, 0f,  50f)]