perf(ui): retain shaped report text

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

@@ -92,6 +92,46 @@ public sealed class CharacterControllerTests
             text => text.EventId == CharacterController.MainTextId);
     }
 
+    [Fact]
+    public void Stable_draws_reuse_report_until_source_or_layout_changes()
+    {
+        ImportedLayout layout = FixtureLoader.LoadCharacterInformation();
+        int builds = 0;
+        int deaths = 1;
+        Action? changed = null;
+        using CharacterInformationUiController controller =
+            CharacterController.Bind(
+                layout,
+                () =>
+                {
+                    builds++;
+                    return new CharacterSheet { Deaths = deaths };
+                },
+                subscribeChanged: handler =>
+                {
+                    changed = handler;
+                    return new TestSubscription();
+                });
+        UiText text = Assert.IsType<UiText>(
+            layout.FindElement(CharacterController.MainTextId));
+
+        IReadOnlyList<UiText.Line> first = text.LinesProvider();
+        IReadOnlyList<UiText.Line> second = text.LinesProvider();
+        Assert.Same(first, second);
+        Assert.Equal(1, builds);
+
+        deaths = 2;
+        changed!();
+        IReadOnlyList<UiText.Line> changedLines = text.LinesProvider();
+        Assert.NotSame(first, changedLines);
+        Assert.Equal(2, builds);
+
+        text.Width -= 24f;
+        IReadOnlyList<UiText.Line> resized = text.LinesProvider();
+        Assert.NotSame(changedLines, resized);
+        Assert.Equal(2, builds);
+    }
+
     private static string Report(CharacterSheet sheet)
         => CharacterController.BuildReport(sheet, CharacterInfoStrings.English);
 
@@ -101,4 +141,11 @@ public sealed class CharacterControllerTests
     private static ImportedLayout FakeLayout()
         => new(new UiPanel { Width = 300f, Height = 362f },
             new Dictionary<uint, UiElement>());
+
+    private sealed class TestSubscription : IDisposable
+    {
+        public void Dispose()
+        {
+        }
+    }
 }

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

@@ -0,0 +1,79 @@
+using AcDream.App.UI;
+using AcDream.App.UI.Layout;
+
+namespace AcDream.App.Tests.UI.Layout;
+
+public sealed class UiTextLayoutCacheTests
+{
+    [Fact]
+    public void Stable_content_and_layout_borrow_same_shaped_lines()
+    {
+        var text = new UiText { Width = 120f };
+        int shapes = 0;
+        var cache = new UiTextLayoutCache<string>(
+            text,
+            (target, value) =>
+            {
+                shapes++;
+                return IndicatorDetailText.Shape(target, value);
+            },
+            "alpha beta gamma",
+            StringComparer.Ordinal);
+
+        IReadOnlyList<UiText.Line> first = cache.GetLines();
+        IReadOnlyList<UiText.Line> second = cache.GetLines();
+
+        Assert.Same(first, second);
+        Assert.Equal(1, shapes);
+
+        cache.SetValue(new string("alpha beta gamma".ToCharArray()));
+        Assert.Same(first, cache.GetLines());
+        Assert.Equal(1, shapes);
+    }
+
+    [Fact]
+    public void Content_width_and_font_inputs_invalidate_shaping()
+    {
+        var text = new UiText { Width = 120f };
+        int shapes = 0;
+        var cache = new UiTextLayoutCache<string>(
+            text,
+            (target, value) =>
+            {
+                shapes++;
+                return IndicatorDetailText.Shape(target, value);
+            },
+            "alpha beta gamma",
+            StringComparer.Ordinal);
+
+        IReadOnlyList<UiText.Line> first = cache.GetLines();
+        cache.SetValue("delta epsilon");
+        IReadOnlyList<UiText.Line> changed = cache.GetLines();
+        text.Width = 48f;
+        IReadOnlyList<UiText.Line> resized = cache.GetLines();
+
+        Assert.NotSame(first, changed);
+        Assert.NotSame(changed, resized);
+        Assert.Equal(3, shapes);
+    }
+
+    [Fact]
+    public void Stable_provider_poll_allocates_no_managed_memory()
+    {
+        var text = new UiText { Width = 120f };
+        var cache = new UiTextLayoutCache<string>(
+            text,
+            static (target, value) => IndicatorDetailText.Shape(target, value),
+            "alpha beta gamma",
+            StringComparer.Ordinal);
+        Func<IReadOnlyList<UiText.Line>> provider = cache.Provider;
+        _ = provider();
+
+        long before = GC.GetAllocatedBytesForCurrentThread();
+        for (int i = 0; i < 1_000; i++)
+            _ = provider();
+        long allocated = GC.GetAllocatedBytesForCurrentThread() - before;
+
+        Assert.Equal(0L, allocated);
+    }
+}