Fix PlaceAbove z-order positioning

src/ModernOverlay.Win32/NativeMethods.cs

@@ -72,6 +72,7 @@ internal static class NativeMethods
     internal const int VkRWin = 0x5C;
 
     internal static readonly nint HwndTopMost = new(-1);
+    internal static readonly nint HwndTop = new(0);
     internal static readonly nint HwndNoTopMost = new(-2);
     internal static readonly nint DpiAwarenessContextPerMonitorAwareV2 = new(-4);
 

src/ModernOverlay.Win32/Win32WindowZOrder.cs

@@ -14,12 +14,23 @@ public static void RemoveTopmost(nint hwnd)
 
     public static void PlaceAbove(nint hwnd, nint hwndInsertAfter)
     {
+        if (!Win32WindowQuery.IsWindow(hwnd))
+        {
+            throw new ArgumentException("The HWND must be a valid window.", nameof(hwnd));
+        }
+
         if (!Win32WindowQuery.IsWindow(hwndInsertAfter))
         {
             throw new ArgumentException("The relative z-order HWND must be a valid window.", nameof(hwndInsertAfter));
         }
 
-        SetZOrder(hwnd, hwndInsertAfter, "SetWindowPos(place above)");
+        nint previousWindow = NativeMethods.GetWindow(hwndInsertAfter, NativeMethods.GwHwndPrev);
+        if (previousWindow == hwnd)
+        {
+            return;
+        }
+
+        SetZOrder(hwnd, previousWindow == 0 ? NativeMethods.HwndTop : previousWindow, "SetWindowPos(place above)");
     }
 
     private static void SetZOrder(nint hwnd, nint insertAfter, string operation)

tests/ModernOverlay.Tests/Win32StyleTests.cs

@@ -98,6 +98,58 @@ public void Win32QueryExposesRequiredWindowMetadata()
         Assert.AreEqual(Win32WindowDisplayAffinity.None, Win32WindowEffects.GetDisplayAffinity(window.Hwnd));
     }
 
+    [TestMethod]
+    [TestCategory("WindowsIntegration")]
+    public void PlaceAbovePositionsWindowImmediatelyAboveRelativeWindow()
+    {
+        using Win32OverlayWindow target = Win32OverlayWindow.Create(new Win32OverlayWindowOptions(
+            ClassName: null,
+            Title: "ModernOverlay z-order target",
+            X: 20,
+            Y: 20,
+            Width: 160,
+            Height: 90,
+            ClickThrough: false,
+            TopMost: false,
+            ToolWindow: true));
+        using Win32OverlayWindow overlay = Win32OverlayWindow.Create(new Win32OverlayWindowOptions(
+            ClassName: null,
+            Title: "ModernOverlay z-order overlay",
+            X: 20,
+            Y: 20,
+            Width: 160,
+            Height: 90,
+            ClickThrough: true,
+            TopMost: false,
+            ToolWindow: true));
+
+        target.Show();
+        overlay.Show();
+
+        Win32WindowZOrder.PlaceAbove(overlay.Hwnd, target.Hwnd);
+
+        Assert.IsTrue(Win32WindowQuery.TryGetPreviousWindow(target.Hwnd, out nint previous));
+        Assert.AreEqual(overlay.Hwnd, previous);
+    }
+
+    [TestMethod]
+    [TestCategory("WindowsIntegration")]
+    public void PlaceAboveRejectsInvalidMovedWindowHandle()
+    {
+        using Win32OverlayWindow target = Win32OverlayWindow.Create(new Win32OverlayWindowOptions(
+            ClassName: null,
+            Title: "ModernOverlay invalid z-order target",
+            X: 20,
+            Y: 20,
+            Width: 160,
+            Height: 90,
+            ClickThrough: false,
+            TopMost: false,
+            ToolWindow: true));
+
+        Assert.ThrowsExactly<ArgumentException>(() => Win32WindowZOrder.PlaceAbove(0, target.Hwnd));
+    }
+
     [TestMethod]
     [TestCategory("WindowsIntegration")]
     public async Task OverlayOptionAppliesCaptureExclusionBeforeUse()