feat(ui): D.2b — toolbar collapse-to-one-row (bottom-edge snap resize hides/shows row 2)

UiElement.MaxHeight + ResizableEdges mask; UiCollapsibleFrame snaps height to the nearer
of {collapsed,expanded} and toggles row-2 visibility; GameWindow computes the two heights
from the layout + top-anchors the content. Amends IA-17. UiNineSlicePanel unsealed to
allow subclassing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -0,0 +1,63 @@
+using AcDream.App.UI;
+using Xunit;
+
+namespace AcDream.App.Tests.UI;
+
+public class UiCollapsibleFrameTests
+{
+    private static UiCollapsibleFrame MakeFrame(out UiPanel row2a, out UiPanel row2b)
+    {
+        var f = new UiCollapsibleFrame(_ => (1u, 1, 1))
+        {
+            CollapsedHeight = 96f,
+            ExpandedHeight  = 128f,
+        };
+        row2a = new UiPanel(); row2b = new UiPanel();
+        f.SecondRow = new UiElement[] { row2a, row2b };
+        return f;
+    }
+
+    [Fact]
+    public void Tick_belowMidpoint_snapsCollapsed_hidesSecondRow()
+    {
+        var f = MakeFrame(out var a, out var b);
+        f.Height = 100f;                 // nearer the collapsed stop (midpoint 112)
+        f.TickForTest(0.016);
+        Assert.Equal(96f, f.Height);
+        Assert.False(a.Visible);
+        Assert.False(b.Visible);
+        Assert.False(f.IsExpanded);
+    }
+
+    [Fact]
+    public void Tick_aboveMidpoint_snapsExpanded_showsSecondRow()
+    {
+        var f = MakeFrame(out var a, out var b);
+        f.Height = 120f;                 // nearer the expanded stop
+        f.TickForTest(0.016);
+        Assert.Equal(128f, f.Height);
+        Assert.True(a.Visible);
+        Assert.True(b.Visible);
+        Assert.True(f.IsExpanded);
+    }
+
+    [Fact]
+    public void Tick_notConfigured_isNoOp()
+    {
+        var f = new UiCollapsibleFrame(_ => (1u, 1, 1)); // Collapsed==Expanded==0
+        f.Height = 50f;
+        f.TickForTest(0.016);
+        Assert.Equal(50f, f.Height);     // unchanged, no divide/no forced height
+    }
+
+    [Fact]
+    public void HitEdges_respectsResizableEdgesMask_bottomOnly()
+    {
+        var panel = new UiPanel { Left = 100, Top = 100, Width = 200, Height = 100,
+            Resizable = true, ResizableEdges = ResizeEdges.Bottom };
+        // bottom edge (y=200) → Bottom only
+        Assert.Equal(ResizeEdges.Bottom, UiRoot.HitEdges(panel, 200, 200, 5));
+        // top edge (y=100) → masked out → None
+        Assert.Equal(ResizeEdges.None, UiRoot.HitEdges(panel, 200, 100, 5));
+    }
+}

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

@@ -157,7 +157,7 @@ public class UiRootInputTests
     public void ResizeRect_RightBottom_GrowsSizeOnly()
     {
         var (x, y, w, h) = UiRoot.ResizeRect(10, 20, 100, 50,
-            ResizeEdges.Right | ResizeEdges.Bottom, dx: 30, dy: 15, minW: 40, minH: 40);
+            ResizeEdges.Right | ResizeEdges.Bottom, dx: 30, dy: 15, minW: 40, minH: 40, maxW: float.MaxValue, maxH: float.MaxValue);
         Assert.Equal(10f, x); Assert.Equal(20f, y);
         Assert.Equal(130f, w); Assert.Equal(65f, h);
     }
@@ -168,7 +168,7 @@ public class UiRootInputTests
         // Drag left edge right by 80 on a 100-wide / min-40 window: width clamps to 40,
         // origin shifts so the RIGHT edge (110) stays put → x = 70.
         var (x, _, w, _) = UiRoot.ResizeRect(10, 20, 100, 50,
-            ResizeEdges.Left, dx: 80, dy: 0, minW: 40, minH: 40);
+            ResizeEdges.Left, dx: 80, dy: 0, minW: 40, minH: 40, maxW: float.MaxValue, maxH: float.MaxValue);
         Assert.Equal(40f, w);
         Assert.Equal(70f, x);
     }