feat(windows): Added event to forward TitleBar HwndProc to avoid adding additional hooks and timing issues

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

@@ -0,0 +1,33 @@
+// This Source Code Form is subject to the terms of the MIT License.
+// If a copy of the MIT was not distributed with this file, You can obtain one at https://opensource.org/licenses/MIT.
+// Copyright (C) Leszek Pomianowski and WPF UI Contributors.
+// All Rights Reserved.
+
+// ReSharper disable once CheckNamespace
+namespace Wpf.Ui.Controls;
+
+public class HwndProcEventArgs : EventArgs
+{
+    public bool Handled { get; set; }
+
+    public IntPtr? ReturnValue { get; set; }
+
+    public bool IsMouseOverDetectedHeaderContent { get; }
+
+    public IntPtr HWND { get; }
+
+    public int Message { get; }
+
+    public IntPtr WParam { get; }
+
+    public IntPtr LParam { get; }
+
+    internal HwndProcEventArgs(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, bool isMouseOverDetectedHeaderContent)
+    {
+        HWND = hwnd;
+        Message = msg;
+        WParam = wParam;
+        LParam = lParam;
+        IsMouseOverDetectedHeaderContent = isMouseOverDetectedHeaderContent;
+    }
+}

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

@@ -37,6 +37,8 @@ public class TitleBar : System.Windows.Controls.Control, IThemeControl
 
     private DependencyObject? _parentWindow;
 
+    public event EventHandler<HwndProcEventArgs>? WndProcInvoked;
+
     /// <summary>Identifies the <see cref="ApplicationTheme"/> dependency property.</summary>
     public static readonly DependencyProperty ApplicationThemeProperty = DependencyProperty.Register(
         nameof(ApplicationTheme),
@@ -682,6 +684,15 @@ or User32.WM.NCLBUTTONUP
             }
         }
 
+        var e = new HwndProcEventArgs(hwnd, msg, wParam, lParam, isMouseOverHeaderContent);
+        WndProcInvoked?.Invoke(this, e);
+
+        if (e.ReturnValue != null)
+        {
+            handled = e.Handled;
+            return e.ReturnValue ?? IntPtr.Zero;
+        }
+
         switch (message)
         {
             case User32.WM.NCHITTEST when CloseWindowByDoubleClickOnIcon && _icon.IsMouseOverElement(lParam):