fix(controls): Fix flickering issue when hovering over the top-right close button

src/Wpf.Ui/Controls/TitleBar/TitleBar.WindowResize.cs

@@ -73,6 +73,15 @@ private IntPtr GetWindowBorderHitTestResult(IntPtr hwnd, IntPtr lParam)
             hit |= 0b1000u; // bottom
 #pragma warning restore
 
+        if (hit == 0b0110u)
+        {
+            const int cornerWidth = 1;
+            if (x < windowRect.right - cornerWidth)
+            {
+                hit = 0b0100u;
+            }
+        }
+
         return hit switch
         {
             0b0101u => (IntPtr)PInvoke.HTTOPLEFT, // top    + left  (0b0100 | 0b0001)

src/Wpf.Ui/Controls/TitleBar/TitleBar.cs

@@ -674,6 +674,134 @@ or PInvoke.WM_NCLBUTTONUP
             return IntPtr.Zero;
         }
 
+        bool isMouseOverHeaderContent = false;
+        bool isMouseOverButtons = false;
+        IntPtr htResult = (IntPtr)PInvoke.HTNOWHERE;
+
+        // For WM_NCHITTEST, perform resize detection first, and skip button hit testing if top-left or top-right corner resize detection succeeds
+        if (message == PInvoke.WM_NCHITTEST)
+        {
+            if (TrailingContent is UIElement || Header is UIElement || CenterContent is UIElement)
+            {
+                UIElement? headerLeftUIElement = Header as UIElement;
+                UIElement? headerCenterUIElement = CenterContent as UIElement;
+                UIElement? headerRightUiElement = TrailingContent as UIElement;
+
+                isMouseOverHeaderContent =
+                    (headerLeftUIElement is not null
+                        && headerLeftUIElement != _titleBlock
+                        && TitleBarButton.IsMouseOverNonClient(headerLeftUIElement, lParam)) || (headerCenterUIElement is not null
+                        && TitleBarButton.IsMouseOverNonClient(headerCenterUIElement, lParam)) || (headerRightUiElement is not null
+                        && TitleBarButton.IsMouseOverNonClient(headerRightUiElement, lParam));
+            }
+
+            TitleBarButton? rightmostButton = null;
+            double rightmostRightEdge = double.MinValue;
+
+            foreach (TitleBarButton button in _buttons)
+            {
+                if (button is null)
+                {
+                    continue;
+                }
+
+                try
+                {
+                    if (PresentationSource.FromVisual(button) is not null)
+                    {
+                        Point buttonTopLeft = button.PointToScreen(new Point(0, 0));
+                        double buttonRightEdge = buttonTopLeft.X + button.RenderSize.Width;
+
+                        if (buttonRightEdge > rightmostRightEdge)
+                        {
+                            rightmostRightEdge = buttonRightEdge;
+                            rightmostButton = button;
+                        }
+                    }
+                }
+                catch
+                {
+                    // Ignore visual transform errors and keep searching.
+                }
+
+                if (TitleBarButton.IsMouseOverNonClient(button, lParam))
+                {
+                    isMouseOverButtons = true;
+                }
+            }
+
+            htResult = GetWindowBorderHitTestResult(hwnd, lParam);
+
+            // If resize hit test succeeds, let Windows handle it
+            if (htResult != (IntPtr)PInvoke.HTNOWHERE)
+            {
+                handled = true;
+                return htResult;
+            }
+
+            if (rightmostButton is not null
+                && Windows.Win32.PInvoke.GetCursorPos(out Windows.Win32.POINT cursorPoint))
+            {
+                Point cursorPosition = new(cursorPoint.X, cursorPoint.Y);
+
+                try
+                {
+                    Point rightmostTopLeft = rightmostButton.PointToScreen(new Point(0, 0));
+                    double rightEdge = rightmostTopLeft.X + rightmostButton.RenderSize.Width;
+                    double leftEdge = rightEdge - 1;
+                    double bottomEdge = rightmostTopLeft.Y + rightmostButton.RenderSize.Height;
+
+                    if (
+                        cursorPosition.X >= leftEdge
+                        && cursorPosition.X <= rightEdge
+                        && cursorPosition.Y >= rightmostTopLeft.Y
+                        && cursorPosition.Y <= bottomEdge
+                    )
+                    {
+                        handled = true;
+                        return (IntPtr)PInvoke.HTRIGHT;
+                    }
+                }
+                catch
+                {
+                    // Ignore transform errors and fall back to default hit testing.
+                }
+            }
+
+            if (isMouseOverButtons)
+            {
+                htResult = (IntPtr)PInvoke.HTNOWHERE;
+            }
+        }
+        else if (message == PInvoke.WM_NCLBUTTONDOWN)
+        {
+            // For WM_NCLBUTTONDOWN, also skip button hit testing if within top-left or top-right corner resize area
+            // This ensures resize handling works correctly
+            foreach (TitleBarButton button in _buttons)
+            {
+                if (button is null)
+                {
+                    continue;
+                }
+
+                if (TitleBarButton.IsMouseOverNonClient(button, lParam))
+                {
+                    isMouseOverButtons = true;
+                    break;
+                }
+            }
+
+            htResult = GetWindowBorderHitTestResult(hwnd, lParam);
+
+            if (htResult != (IntPtr)PInvoke.HTNOWHERE)
+            {
+                // If within resize area, skip button hit testing
+                // and let Windows handle the default resize processing
+                handled = false;
+                return IntPtr.Zero;
+            }
+        }
+
         foreach (TitleBarButton button in _buttons)
         {
             if (!button.ReactToHwndHook(message, lParam, out IntPtr returnIntPtr))
@@ -699,25 +827,6 @@ or PInvoke.WM_NCLBUTTONUP
             return returnIntPtr;
         }
 
-        bool isMouseOverHeaderContent = false;
-        IntPtr htResult = (IntPtr)PInvoke.HTNOWHERE;
-
-        if (message == PInvoke.WM_NCHITTEST)
-        {
-            if (TrailingContent is UIElement || Header is UIElement || CenterContent is UIElement)
-            {
-                UIElement? headerLeftUIElement = Header as UIElement;
-                UIElement? headerCenterUIElement = CenterContent as UIElement;
-                UIElement? headerRightUiElement = TrailingContent as UIElement;
-
-                isMouseOverHeaderContent = (headerLeftUIElement is not null && headerLeftUIElement != _titleBlock && headerLeftUIElement.IsMouseOverElement(lParam))
-                    || (headerCenterUIElement?.IsMouseOverElement(lParam) ?? false)
-                    || (headerRightUiElement?.IsMouseOverElement(lParam) ?? false);
-            }
-
-            htResult = GetWindowBorderHitTestResult(hwnd, lParam);
-        }
-
         var e = new HwndProcEventArgs(hwnd, msg, wParam, lParam, isMouseOverHeaderContent);
         WndProcInvoked?.Invoke(this, e);
 
@@ -729,14 +838,14 @@ or PInvoke.WM_NCLBUTTONUP
 
         switch (message)
         {
-            case PInvoke.WM_NCHITTEST when CloseWindowByDoubleClickOnIcon && _icon.IsMouseOverElement(lParam):
+            case PInvoke.WM_NCHITTEST when CloseWindowByDoubleClickOnIcon && TitleBarButton.IsMouseOverNonClient(_icon, lParam):
                 // Ideally, clicking on the icon should open the system menu, but when the system menu is opened manually, double-clicking on the icon does not close the window
                 handled = true;
                 return (IntPtr)PInvoke.HTSYSMENU;
             case PInvoke.WM_NCHITTEST when htResult != (IntPtr)PInvoke.HTNOWHERE:
                 handled = true;
                 return htResult;
-            case PInvoke.WM_NCHITTEST when this.IsMouseOverElement(lParam) && !isMouseOverHeaderContent:
+            case PInvoke.WM_NCHITTEST when TitleBarButton.IsMouseOverNonClient(this, lParam) && !isMouseOverHeaderContent:
                 handled = true;
                 return (IntPtr)PInvoke.HTCAPTION;
             default:

src/Wpf.Ui/Controls/TitleBar/TitleBarButton.cs

@@ -12,6 +12,92 @@ namespace Wpf.Ui.Controls;
 
 public class TitleBarButton : Wpf.Ui.Controls.Button
 {
+    // We intentionally keep this logic local to TitleBar components to avoid changing
+    // global hit-testing behavior for other controls.
+    internal static bool IsMouseOverNonClient(UIElement element, IntPtr lParam, double tolerance = 1.0)
+    {
+        // This will be invoked very often and must be as simple as possible.
+        if (lParam == IntPtr.Zero)
+        {
+            return false;
+        }
+
+        try
+        {
+            // Ensure the visual is connected to a presentation source (needed for PointFromScreen).
+            if (PresentationSource.FromVisual(element) == null)
+            {
+                return false;
+            }
+
+            var mousePosition = TryGetCursorPos(out Point cursorPosition)
+                ? cursorPosition
+                : GetLParamPoint(lParam);
+
+            // Add a small tolerance to reduce hover flicker at pixel boundaries (rounding/DPI edge cases).
+            var hitRect = new Rect(
+                -tolerance,
+                -tolerance,
+                element.RenderSize.Width + (2 * tolerance),
+                element.RenderSize.Height + (2 * tolerance)
+            );
+
+            if (!hitRect.Contains(element.PointFromScreen(mousePosition)) || !element.IsHitTestVisible)
+            {
+                return false;
+            }
+
+            // If element is Panel, check if children at mousePosition is with IsHitTestVisible false.
+            if (element is System.Windows.Controls.Panel panel)
+            {
+                foreach (UIElement child in panel.Children)
+                {
+                    var childHitRect = new Rect(
+                        -tolerance,
+                        -tolerance,
+                        child.RenderSize.Width + (2 * tolerance),
+                        child.RenderSize.Height + (2 * tolerance)
+                    );
+
+                    if (childHitRect.Contains(child.PointFromScreen(mousePosition)))
+                    {
+                        return child.IsHitTestVisible;
+                    }
+                }
+
+                return false;
+            }
+
+            return true;
+        }
+        catch
+        {
+            return false;
+        }
+    }
+
+    private static Point GetLParamPoint(IntPtr lParam)
+    {
+        long lp = lParam.ToInt64();
+        int x = (short)(lp & 0xFFFF);
+        int y = (short)(lp >> 16);
+
+        return new Point(x, y);
+    }
+
+    private static bool TryGetCursorPos(out Point mousePosition)
+    {
+        mousePosition = default;
+
+        if (!Windows.Win32.PInvoke.GetCursorPos(out Windows.Win32.POINT point))
+        {
+            return false;
+        }
+
+        mousePosition = new Point(point.X, point.Y);
+        return true;
+    }
+
     /// <summary>Identifies the <see cref="ButtonType"/> dependency property.</summary>
     public static readonly DependencyProperty ButtonTypeProperty = DependencyProperty.Register(
         nameof(ButtonType),
@@ -179,7 +265,7 @@ internal bool ReactToHwndHook(uint msg, IntPtr lParam, out IntPtr returnIntPtr)
         switch (msg)
         {
             case PInvoke.WM_NCHITTEST:
-                if (this.IsMouseOverElement(lParam))
+                if (IsMouseOverNonClient(this, lParam))
                 {
                     /*Debug.WriteLine($"Hitting {ButtonType} | return code {_returnValue}");*/
                     Hover();
@@ -192,10 +278,10 @@ internal bool ReactToHwndHook(uint msg, IntPtr lParam, out IntPtr returnIntPtr)
             case PInvoke.WM_NCMOUSELEAVE: // Mouse leaves the window
                 RemoveHover();
                 return false;
-            case PInvoke.WM_NCLBUTTONDOWN when this.IsMouseOverElement(lParam): // Left button clicked down
+            case PInvoke.WM_NCLBUTTONDOWN when IsMouseOverNonClient(this, lParam): // Left button clicked down
                 _isClickedDown = true;
                 return true;
-            case PInvoke.WM_NCLBUTTONUP when _isClickedDown && this.IsMouseOverElement(lParam): // Left button clicked up
+            case PInvoke.WM_NCLBUTTONUP when _isClickedDown && IsMouseOverNonClient(this, lParam): // Left button clicked up
                 InvokeClick();
                 return true;
             default:

src/Wpf.Ui/Interop/PInvoke.cs

@@ -4,16 +4,28 @@
 // All Rights Reserved.
 
 using System.Runtime.InteropServices;
-using Windows.Win32.Foundation;
 using Windows.Win32.UI.WindowsAndMessaging;
 
 namespace Windows.Win32;
 
 internal static partial class PInvoke
 {
     [
         DllImport("USER32.dll", ExactSpelling = true, EntryPoint = "SetWindowLongPtrW", SetLastError = true),
         DefaultDllImportSearchPaths(DllImportSearchPath.System32)
     ]
-    internal static extern nint SetWindowLongPtr(HWND hWnd, WINDOW_LONG_PTR_INDEX nIndex, nint dwNewLong);
+    internal static extern nint SetWindowLongPtr(Windows.Win32.Foundation.HWND hWnd, WINDOW_LONG_PTR_INDEX nIndex, nint dwNewLong);
+
+    [
+        DllImport("USER32.dll", ExactSpelling = true, EntryPoint = "GetCursorPos", SetLastError = true),
+        DefaultDllImportSearchPaths(DllImportSearchPath.System32)
+    ]
+    internal static extern bool GetCursorPos(out POINT lpPoint);
+}
+
+[StructLayout(LayoutKind.Sequential)]
+internal struct POINT
+{
+    public int X;
+    public int Y;
 }