feat(ui): port retained widget foundations

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

@@ -0,0 +1,132 @@
+using AcDream.App.UI;
+
+namespace AcDream.App.Tests.UI;
+
+public sealed class UiLayoutPolicyTests
+{
+    private static readonly UiPixelRect OriginalParent =
+        UiPixelRect.FromPositionAndSize(0, 0, 800, 600);
+    private static readonly UiPixelRect OriginalChild =
+        UiPixelRect.FromPositionAndSize(100, 50, 200, 100);
+
+    [Fact]
+    public void ModeZero_PreservesEveryCurrentEdge()
+    {
+        var current = new UiPixelRect(111, 66, 333, 177);
+        var resizedParent = UiPixelRect.FromPositionAndSize(0, 0, 1600, 900);
+
+        var result = UiLayoutPolicy.Apply(
+            0, 0, 0, 0,
+            OriginalChild,
+            OriginalParent,
+            current,
+            resizedParent);
+
+        Assert.Equal(current, result);
+    }
+
+    [Fact]
+    public void ModeOne_KeepsNearEdgesAndAddsParentDeltaToFarEdges()
+    {
+        var resizedParent = UiPixelRect.FromPositionAndSize(0, 0, 1000, 700);
+
+        var result = UiLayoutPolicy.Apply(
+            1, 1, 1, 1,
+            OriginalChild,
+            OriginalParent,
+            OriginalChild,
+            resizedParent);
+
+        Assert.Equal(new UiPixelRect(100, 50, 499, 249), result);
+        Assert.Equal(400, result.Width);
+        Assert.Equal(200, result.Height);
+    }
+
+    [Fact]
+    public void MixedModeTwoAndOne_MovesFixedSizeChildWithFarSide()
+    {
+        var resizedParent = UiPixelRect.FromPositionAndSize(0, 0, 1000, 700);
+
+        var result = UiLayoutPolicy.Apply(
+            2, 2, 1, 1,
+            OriginalChild,
+            OriginalParent,
+            OriginalChild,
+            resizedParent);
+
+        Assert.Equal(new UiPixelRect(300, 150, 499, 249), result);
+        Assert.Equal(OriginalChild.Width, result.Width);
+        Assert.Equal(OriginalChild.Height, result.Height);
+    }
+
+    [Fact]
+    public void ModeThree_UsesInclusivePixelCenterFormula()
+    {
+        var resizedParent = UiPixelRect.FromPositionAndSize(0, 0, 1001, 701);
+
+        var result = UiLayoutPolicy.Apply(
+            3, 3, 3, 3,
+            OriginalChild,
+            OriginalParent,
+            OriginalChild,
+            resizedParent);
+
+        Assert.Equal(new UiPixelRect(400, 300, 599, 399), result);
+    }
+
+    [Fact]
+    public void ModeFour_ScalesEachCoordinateWithTruncationTowardZero()
+    {
+        var originalParent = UiPixelRect.FromPositionAndSize(0, 0, 10, 10);
+        var originalChild = new UiPixelRect(-3, -3, -1, -1);
+        var resizedParent = UiPixelRect.FromPositionAndSize(0, 0, 15, 15);
+
+        var result = UiLayoutPolicy.Apply(
+            4, 4, 4, 4,
+            originalChild,
+            originalParent,
+            originalChild,
+            resizedParent);
+
+        Assert.Equal(new UiPixelRect(-4, -4, -1, -1), result);
+    }
+
+    [Fact]
+    public void AssigningCompatibilityAnchors_OptsOutOfImportedPolicy()
+    {
+        var element = new UiPanel
+        {
+            LayoutPolicy = new UiLayoutPolicy(
+                1, 1, 1, 1,
+                OriginalChild,
+                OriginalParent),
+        };
+
+        element.Anchors = AnchorEdges.Left | AnchorEdges.Top;
+
+        Assert.Null(element.LayoutPolicy);
+    }
+
+    [Fact]
+    public void ApplyAnchor_UsesImportedPolicyInsteadOfCompatibilityMargins()
+    {
+        var element = new UiPanel
+        {
+            Left = 100,
+            Top = 50,
+            Width = 200,
+            Height = 100,
+            LayoutPolicy = new UiLayoutPolicy(
+                1, 1, 1, 1,
+                OriginalChild,
+                OriginalParent),
+        };
+
+        element.ApplyAnchor(1000, 700);
+
+        Assert.Equal(100f, element.Left);
+        Assert.Equal(50f, element.Top);
+        Assert.Equal(400f, element.Width);
+        Assert.Equal(200f, element.Height);
+    }
+}