Properly handle nc hittest for caption buttons (#17380)

* Set e.Handle=true on caption button click event

* Return HTCLOSE, HTMINBUTTON and HTMAXBUTTON results on corresponding caption buttons

* Redirect fake-client input for caption buttons

* Introduce Win32Properties.NonClientHitTestResultProperty instead of hardcoding

* Changes after revie

src/Avalonia.Controls/Chrome/CaptionButtons.cs

@@ -15,10 +15,10 @@ namespace Avalonia.Controls.Chrome
     [PseudoClasses(":minimized", ":normal", ":maximized", ":fullscreen")]
     public class CaptionButtons : TemplatedControl
     {
-        private const string PART_CloseButton = "PART_CloseButton";
+        internal const string PART_CloseButton = "PART_CloseButton";
-        private const string PART_RestoreButton = "PART_RestoreButton";
+        internal const string PART_RestoreButton = "PART_RestoreButton";
-        private const string PART_MinimizeButton = "PART_MinimizeButton";
+        internal const string PART_MinimizeButton = "PART_MinimizeButton";
-        private const string PART_FullScreenButton = "PART_FullScreenButton";
+        internal const string PART_FullScreenButton = "PART_FullScreenButton";
 
         private Button? _restoreButton;
         private IDisposable? _disposables;
@@ -102,24 +102,40 @@ namespace Avalonia.Controls.Chrome
             
             if (e.NameScope.Find<Button>(PART_CloseButton) is { } closeButton)
             {
-                closeButton.Click += (sender, e) => OnClose();
+                closeButton.Click += (_, args) =>
+                {
+                    OnClose();
+                    args.Handled = true;
+                };
             }
 
             if (e.NameScope.Find<Button>(PART_RestoreButton) is { } restoreButton)
             {
-                restoreButton.Click += (sender, e) => OnRestore();
+                restoreButton.Click += (_, args) =>
+                {
+                    OnRestore();
+                    args.Handled = true;
+                };
                 restoreButton.IsEnabled = HostWindow?.CanResize ?? true;
                 _restoreButton = restoreButton;
             }
             
             if (e.NameScope.Find<Button>(PART_MinimizeButton) is { } minimizeButton)
             {
-                minimizeButton.Click += (sender, e) => OnMinimize();
+                minimizeButton.Click += (_, args) =>
+                {
+                    OnMinimize();
+                    args.Handled = true;
+                };
             }
             
             if (e.NameScope.Find<Button>(PART_FullScreenButton) is { } fullScreenButton)
             {
-                fullScreenButton.Click += (sender, e) => OnToggleFullScreen();
+                fullScreenButton.Click += (_, args) =>
+                {
+                    OnToggleFullScreen();
+                    args.Handled = true;
+                };
             }
         }
     }

src/Avalonia.Controls/Platform/Win32Properties.cs

@@ -5,6 +5,7 @@ using System.Text;
 using System.Threading.Tasks;
 using Avalonia.Controls;
 using Avalonia.Controls.Platform;
+using Avalonia.Input;
 using Avalonia.Metadata;
 using Avalonia.Platform;
 using static Avalonia.Controls.Platform.IWin32OptionsTopLevelImpl;
@@ -70,5 +71,33 @@ namespace Avalonia.Controls
                 toplevelImpl.WndProcHookCallback -= callback;
             }
         }
+
+        public static readonly AttachedProperty<Win32HitTestValue> NonClientHitTestResultProperty =
+            AvaloniaProperty.RegisterAttached<Visual, Win32HitTestValue>(
+                "NonClientHitTestResult",
+                typeof(Win32Properties),
+                inherits: true,
+                defaultValue: Win32HitTestValue.Client);
+
+        public static void SetNonClientHitTestResult(Visual obj, Win32HitTestValue value) => obj.SetValue(NonClientHitTestResultProperty, value);
+        public static Win32HitTestValue GetNonClientHitTestResult(Visual obj) => obj.GetValue(NonClientHitTestResultProperty);
+
+        public enum Win32HitTestValue
+        {
+            Nowhere = 0,
+            Client = 1,
+            Caption = 2,
+            MinButton = 8,
+            MaxButton = 9,
+            Left = 10,
+            Right = 11,
+            Top = 12,
+            TopLeft = 13,
+            TopRight = 14,
+            Bottom = 15,
+            BottomLeft = 16,
+            BottomRight = 17,
+            Close = 20,
+        }
     }
 }

src/Avalonia.Themes.Fluent/Controls/CaptionButtons.xaml

@@ -62,7 +62,8 @@
           </Button>
           <Button x:Name="PART_MinimizeButton"
                   Theme="{StaticResource FluentCaptionButton}"
-                  AutomationProperties.Name="Minimize">
+                  AutomationProperties.Name="Minimize"
+                  Win32Properties.NonClientHitTestResult="MinButton">
             <Viewbox Width="11" Margin="2">
               <Path Stretch="UniformToFill"
                     Fill="{TemplateBinding Foreground}"
@@ -71,7 +72,8 @@
           </Button>
           <Button x:Name="PART_RestoreButton"
                   Theme="{StaticResource FluentCaptionButton}"
-                  AutomationProperties.Name="Maximize">
+                  AutomationProperties.Name="Maximize"
+                  Win32Properties.NonClientHitTestResult="MaxButton">
             <Viewbox Width="11" Margin="2">
               <Viewbox.RenderTransform>
                 <RotateTransform Angle="-90" />
@@ -86,7 +88,8 @@
                   Background="#ffe81123"
                   BorderBrush="#fff1707a"
                   Theme="{StaticResource FluentCaptionButton}"
-                  AutomationProperties.Name="Close">
+                  AutomationProperties.Name="Close"
+                  Win32Properties.NonClientHitTestResult="Close">
             <Viewbox Width="11" Margin="2">
               <Path Stretch="UniformToFill"
                     Fill="{TemplateBinding Foreground}"

src/Avalonia.Themes.Fluent/Controls/TitleBar.xaml

@@ -22,11 +22,13 @@
             <Panel x:Name="PART_Container">
               <Border x:Name="PART_Background"
                       Background="{TemplateBinding Background}"
-                      IsHitTestVisible="False"/>
+                      IsHitTestVisible="False"
+                      Win32Properties.NonClientHitTestResult="Caption" />
               <CaptionButtons x:Name="PART_CaptionButtons"
                               VerticalAlignment="Top" 
                               HorizontalAlignment="Right"
-                              Foreground="{TemplateBinding Foreground}" />
+                              Foreground="{TemplateBinding Foreground}"
+                              Win32Properties.NonClientHitTestResult="Client" />
             </Panel>
           </Panel>
         </ControlTemplate>

src/Avalonia.Themes.Simple/Controls/CaptionButtons.xaml

@@ -66,7 +66,8 @@
           </Button>
           <Button x:Name="PART_MinimizeButton"
                   Theme="{StaticResource SimpleCaptionButton}"
-                  AutomationProperties.Name="Minimize">
+                  AutomationProperties.Name="Minimize"
+                  Win32Properties.NonClientHitTestResult="MinButton">
             <Viewbox Width="11"
                      Margin="2">
               <Path Data="M2048 1229v-205h-2048v205h2048z"
@@ -76,7 +77,8 @@
           </Button>
           <Button x:Name="PART_RestoreButton"
                   Theme="{StaticResource SimpleCaptionButton}"
-                  AutomationProperties.Name="Maximize">
+                  AutomationProperties.Name="Maximize"
+                  Win32Properties.NonClientHitTestResult="MaxButton">
             <Viewbox Width="11"
                      Margin="2">
               <Viewbox.RenderTransform>
@@ -92,7 +94,8 @@
                   Background="#ffe81123"
                   BorderBrush="#fff1707a"
                   Theme="{StaticResource SimpleCaptionButton}"
-                  AutomationProperties.Name="Close">
+                  AutomationProperties.Name="Close"
+                  Win32Properties.NonClientHitTestResult="Close">
             <Viewbox Width="11"
                      Margin="2">
               <Path Data="M1169 1024l879 -879l-145 -145l-879 879l-879 -879l-145 145l879 879l-879 879l145 145l879 -879l879 879l145 -145z"

src/Avalonia.Themes.Simple/Controls/TitleBar.xaml

@@ -24,11 +24,14 @@
                  VerticalAlignment="Top" />
           <Panel x:Name="PART_Container">
             <Border x:Name="PART_Background"
-                    Background="{TemplateBinding Background}" />
+                    Background="{TemplateBinding Background}"
+                    IsHitTestVisible="False"
+                    Win32Properties.NonClientHitTestResult="Caption" />
             <CaptionButtons x:Name="PART_CaptionButtons"
                             HorizontalAlignment="Right"
                             VerticalAlignment="Top"
-                            Foreground="{TemplateBinding Foreground}" />
+                            Foreground="{TemplateBinding Foreground}"
+                            Win32Properties.NonClientHitTestResult="Client" />
           </Panel>
         </Panel>
       </ControlTemplate>

src/Windows/Avalonia.Win32/WindowImpl.CustomCaptionProc.cs

@@ -1,6 +1,7 @@
 using System;
 using Avalonia.Controls;
 using Avalonia.Input;
+using Avalonia.Input.Raw;
 using Avalonia.VisualTree;
 using static Avalonia.Win32.Interop.UnmanagedMethods;
 
@@ -8,7 +9,6 @@ namespace Avalonia.Win32
 {
     internal partial class WindowImpl
     {
-        // Hit test the frame for resizing and moving.
         private HitTestValues HitTestNCA(IntPtr hWnd, IntPtr wParam, IntPtr lParam)
         {
             // Get the point coordinates for the hit test (screen space).
@@ -65,9 +65,10 @@ namespace Avalonia.Win32
                 uRow = 2;
             }
 
+            var captionAreaHitTest = WindowState == WindowState.FullScreen ? HitTestValues.HTNOWHERE : HitTestValues.HTCAPTION;
             ReadOnlySpan<HitTestValues> hitZones = stackalloc HitTestValues[]
             {
-                HitTestValues.HTTOPLEFT, onResizeBorder ? HitTestValues.HTTOP : HitTestValues.HTCAPTION,
+                HitTestValues.HTTOPLEFT, onResizeBorder ? HitTestValues.HTTOP : captionAreaHitTest,
                 HitTestValues.HTTOPRIGHT, HitTestValues.HTLEFT, HitTestValues.HTNOWHERE, HitTestValues.HTRIGHT,
                 HitTestValues.HTBOTTOMLEFT, HitTestValues.HTBOTTOM, HitTestValues.HTBOTTOMRIGHT
             };
@@ -79,6 +80,7 @@ namespace Avalonia.Win32
 
         protected virtual IntPtr CustomCaptionProc(IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam, ref bool callDwp)
         {
+            RawPointerEventArgs? e = null;
             IntPtr lRet = IntPtr.Zero;
 
             callDwp = !DwmDefWindowProc(hWnd, msg, wParam, lParam, ref lRet);
@@ -92,47 +94,121 @@ namespace Avalonia.Win32
                 case WindowsMessage.WM_NCHITTEST:
                     if (lRet == IntPtr.Zero)
                     {
-                        if(WindowState == WindowState.FullScreen)
-                        {
-                            return (IntPtr)HitTestValues.HTCLIENT;
-                        }
                         var hittestResult = HitTestNCA(hWnd, wParam, lParam);
-
+                        if (hittestResult is HitTestValues.HTNOWHERE or HitTestValues.HTCAPTION)
-                        lRet = (IntPtr)hittestResult;
-
-                        if (hittestResult == HitTestValues.HTCAPTION)
                         {
-                            var position = PointToClient(PointFromLParam(lParam));
+                            var visualHittestResult = HitTestVisual(lParam);
-
+                            if (visualHittestResult != HitTestValues.HTNOWHERE)
-                            if (_owner is Window window)
                             {
-                                var visual = window.GetVisualAt(position, x =>
+                                hittestResult = visualHittestResult;
-                                {
-                                    if (x is IInputElement ie && (!ie.IsHitTestVisible || !ie.IsEffectivelyVisible))
-                                    {
-                                        return false;
-                                    }
-
-                                    return true;
-                                });
-
-                                if (visual != null)
-                                {
-                                    hittestResult = HitTestValues.HTCLIENT;
-                                    lRet = (IntPtr)hittestResult;
-                                }
                             }
                         }
 
                         if (hittestResult != HitTestValues.HTNOWHERE)
                         {
+                            lRet = (IntPtr)hittestResult;
                             callDwp = false;
                         }
                     }
                     break;
+
+                // Normally, Avalonia doesn't handles non-client input as a special NonClientLeftButtonDown, ignoring move and up events.
+                // What makes it a problem, Avalonia has to mark templated caption buttons as a non-client area.
+                // Meaning, these buttons no longer can accept normal client input.
+                // These messages are needed to explicitly fake this normal client input from non-client messages.
+                // For both WM_NCMOUSE and WM_NCPOINTERUPDATE
+                case WindowsMessage.WM_NCMOUSEMOVE when !IsMouseInPointerEnabled:
+                case WindowsMessage.WM_NCLBUTTONDOWN when !IsMouseInPointerEnabled:
+                case WindowsMessage.WM_NCLBUTTONUP when !IsMouseInPointerEnabled:
+                    if (lRet == IntPtr.Zero
+                        && ShouldRedirectNonClientInput(hWnd, wParam, lParam))
+                    {
+                        e = new RawPointerEventArgs(
+                            _mouseDevice,
+                            unchecked((uint)GetMessageTime()),
+                            Owner,
+                            (WindowsMessage)msg switch
+                            {
+                                WindowsMessage.WM_NCMOUSEMOVE => RawPointerEventType.Move,
+                                WindowsMessage.WM_NCLBUTTONDOWN => RawPointerEventType.LeftButtonDown,
+                                WindowsMessage.WM_NCLBUTTONUP => RawPointerEventType.LeftButtonUp,
+                                _ => throw new ArgumentOutOfRangeException(nameof(msg), msg, null)
+                            },
+                            PointToClient(PointFromLParam(lParam)),
+                            RawInputModifiers.None);
+                    }
+                    break;
+                case WindowsMessage.WM_NCPOINTERUPDATE when _wmPointerEnabled:
+                case WindowsMessage.WM_NCPOINTERDOWN when _wmPointerEnabled:
+                case WindowsMessage.WM_NCPOINTERUP when _wmPointerEnabled:
+                    if (lRet == IntPtr.Zero
+                        && ShouldRedirectNonClientInput(hWnd, wParam, lParam))
+                    {
+                        uint timestamp = 0;
+                        GetDevicePointerInfo(wParam, out var device, out var info, out var point, out var modifiers, ref timestamp);
+                        var eventType = (WindowsMessage)msg switch
+                        {
+                            WindowsMessage.WM_NCPOINTERUPDATE => RawPointerEventType.Move,
+                            WindowsMessage.WM_NCPOINTERDOWN => RawPointerEventType.LeftButtonDown,
+                            WindowsMessage.WM_NCPOINTERUP => RawPointerEventType.LeftButtonUp,
+                            _ => throw new ArgumentOutOfRangeException(nameof(msg), msg, null)
+                        };
+                        e = CreatePointerArgs(device, timestamp, eventType, point, modifiers, info.pointerId);
+                    }
+                    break;
+            }
+
+            if (e is not null && Input is not null)
+            {
+                Input(e);
+                if (e.Handled)
+                {
+                    callDwp = false;
+                    return IntPtr.Zero;
+                }
             }
 
             return lRet;
         }
+
+        private HitTestValues HitTestVisual(IntPtr lParam)
+        {
+            var position = PointToClient(PointFromLParam(lParam));
+            if (_owner is Window window)
+            {
+                var visual = window.GetVisualAt(position, x =>
+                {
+                    if (x is IInputElement ie && (!ie.IsHitTestVisible || !ie.IsEffectivelyVisible))
+                    {
+                        return false;
+                    }
+
+                    return true;
+                });
+
+                if (visual != null)
+                {
+                    var hitTest = Win32Properties.GetNonClientHitTestResult(visual);
+                    return (HitTestValues)hitTest;
+                }
+            }
+            return HitTestValues.HTNOWHERE;
+        }
+
+        private bool ShouldRedirectNonClientInput(IntPtr hWnd, IntPtr wParam, IntPtr lParam)
+        {
+            // We touched frame borders or caption, don't redirect.
+            if (HitTestNCA(hWnd, wParam, lParam) is not (HitTestValues.HTNOWHERE or HitTestValues.HTCAPTION))
+                return false;
+
+            // Redirect only for buttons.
+            return HitTestVisual(lParam)
+                is HitTestValues.HTMINBUTTON
+                or HitTestValues.HTMAXBUTTON
+                or HitTestValues.HTCLOSE
+                or HitTestValues.HTHELP
+                or HitTestValues.HTMENU
+                or HitTestValues.HTSYSMENU;
+        }
     }
 }