fix(ui): #372 — Options tabs no longer blank; UiTemplateListBox viewport fills its ListBox

ROOT CAUSE (proven, not guessed): the lazily-created row viewport was
constructed 0x0 with Left|Top|Right|Bottom fill-anchors. Its first
ApplyAnchor captured mR = parentW - (0+0) = parentW, so
ComputeAnchoredRect's l&&r branch (w = parentW - mR - mL) kept it 0x0
forever. A 0-tall viewport makes UiScrollablePanel.LayoutScrollableChildren
cull every row, so Character/Chat/Config rendered blank while Gameplay
(no viewport — authored static children sized at Build) worked. This is
the exact Gameplay-vs-rest split the user's first connected gate found.

FIX: seed the viewport to the ListBox's current extent at creation, so the
fill-anchor baseline is mR = parentW - parentW = 0 and the viewport tracks
the parent. The ListBox is a static dat child sized at Build, so its extent
is authored by the time the viewport is lazily created during Bind.
Dormancy preserved — the viewport is still created only on the first row.

Reproduced RED then GREEN by UiTemplateListBoxViewportTests (viewport fills;
rows stay visible after the anchor+cull layout pass) — the layout path the
whole fixture conformance suite structurally never drove, which is why
every OP2-OP6 test was green over a live-only blank-tab failure. Full
Release suite 13,131 / 4 skips / 0 failed.

Still owed (NOT fixed here, no evidence yet): the 'only Exit Game worked'
Gameplay-buttons observation needs a re-gate (two buttons are correctly
INERT; the other four have dialog/chat effects that may have gone
unnoticed); and the 13 ID_ChatOption_TextFilter_* labels fail to resolve
(blank captions, behaviour unaffected). See #372.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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

@@ -0,0 +1,79 @@
+using AcDream.App.UI;
+using AcDream.App.UI.Layout;
+
+namespace AcDream.App.Tests.UI;
+
+/// <summary>
+/// #372 regression: the Options panel's Character/Chat/Config tabs rendered
+/// BLANK at the first connected gate while Gameplay (which has no ListBox)
+/// worked. Root cause: <see cref="UiTemplateListBox"/> creates its row
+/// viewport at 0×0 with Left|Top|Right|Bottom fill-anchors; the anchor
+/// baseline captured from that 0-size rect makes
+/// <c>ComputeAnchoredRect</c> keep it 0×0 forever (mR=parentW, so
+/// w=parentW-mR-mL=0), and <see cref="UiScrollablePanel.LayoutScrollableChildren"/>
+/// then culls every row (top+height ≤ 0 is false). These tests pin that the
+/// viewport fills its ListBox and the rows stay visible after a layout pass.
+/// </summary>
+public sealed class UiTemplateListBoxViewportTests
+{
+    private static UiTemplateListBox MakeListBox(float width, float height)
+    {
+        // A ListBox element authoring one template entry, sized like the
+        // Options-panel Character ListBox (0x100001FA, 276×560 authored).
+        var info = new ElementInfo { Id = 0x100001FAu, Type = 5u };
+        var box = new UiTemplateListBox(
+            info,
+            _ => (0u, 0, 0),
+            new[] { new UiTemplateListEntry(0x2100002Bu, 0x10000218u) },
+            scrollbarElementId: 0x100001FBu)
+        {
+            Width = width,
+            Height = height,
+        };
+        return box;
+    }
+
+    [Fact]
+    public void Viewport_FillsTheListBox_NotCollapsedToZero()
+    {
+        var box = MakeListBox(276f, 560f);
+        // Add a couple of rows via the prebuilt path (no DAT needed).
+        box.AddPrebuiltRow(new UiText { Width = 260f, Height = 20f });
+        box.AddPrebuiltRow(new UiText { Width = 260f, Height = 20f });
+
+        UiScrollablePanel? viewport = box.ViewportForTest;
+        Assert.NotNull(viewport);
+
+        // Drive the per-frame layout the draw traversal runs: the parent
+        // ListBox re-anchors its children (UiElement.DrawSelfAndChildren
+        // line ~500), sizing the fill-anchored viewport.
+        viewport!.ApplyAnchor(box.Width, box.Height);
+
+        // The whole point of #372: the viewport must FILL the ListBox, not
+        // collapse. Pre-fix this was 0×0.
+        Assert.Equal(276f, viewport.Width, 3);
+        Assert.Equal(560f, viewport.Height, 3);
+    }
+
+    [Fact]
+    public void Rows_StayVisible_AfterLayout_WhenTheyFitTheViewport()
+    {
+        var box = MakeListBox(276f, 560f);
+        var row0 = new UiText { Width = 260f, Height = 20f };
+        var row1 = new UiText { Width = 260f, Height = 20f };
+        var row2 = new UiText { Width = 260f, Height = 20f };
+        box.AddPrebuiltRow(row0);
+        box.AddPrebuiltRow(row1);
+        box.AddPrebuiltRow(row2);
+
+        UiScrollablePanel viewport = box.ViewportForTest!;
+        viewport.ApplyAnchor(box.Width, box.Height); // size the viewport
+        viewport.LayoutScrollableChildren();          // the cull pass (runs each OnDraw)
+
+        // All three rows sit at y 0/20/40 inside a 560px viewport → visible.
+        // Pre-fix the viewport was 0px tall and every row was culled → blank tab.
+        Assert.True(row0.Visible, "row 0 culled — the #372 blank-tab bug");
+        Assert.True(row1.Visible, "row 1 culled — the #372 blank-tab bug");
+        Assert.True(row2.Visible, "row 2 culled — the #372 blank-tab bug");
+    }
+}