IsMouseOver returns false over some elements in a DockPanel

Question

I'm trying to detect when mouse enters VS 2017 title bar, but I've noticed that MouseEnter and MouseLeave events don't work correctly. Event fires only when mouse enters child controls outlined by green rectangle on the screenshot below.

The title bar is a DockPanel with some elements in it. I've set its background to SolidColorBrush(Colors.Red) to make sure hit test runs correctly. When mouse is over elements in green rectangle IsMouseOver correctly returns true, but everywhere else it is false. For menu bar, IsMouseOver and MouseEnter and MouseLeave events work correctly. What could be wrong there?

Update 2: It is likely that title bar is marked as non-client area and this is what causes this problem

Update:

Here is Visual Tree of main VS window:

Decompiled MainWindowTitleBar class:

using Microsoft.VisualStudio.PlatformUI.Shell.Controls;
using Microsoft.VisualStudio.Shell;
using System;
using System.Windows;
using System.Windows.Automation.Peers;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Interop;
using System.Windows.Media;

namespace Microsoft.VisualStudio.PlatformUI
{
    public sealed class MainWindowTitleBar : Border, INonClientArea
    {
        protected override HitTestResult HitTestCore(PointHitTestParameters hitTestParameters)
        {
            return new PointHitTestResult(this, hitTestParameters.HitPoint);
        }

        int INonClientArea.HitTest(Point point)
        {
            return 2;
        }

        protected override AutomationPeer OnCreateAutomationPeer()
        {
            return new MainWindowTitleBarAutomationPeer(this);
        }

        protected override void OnContextMenuOpening(ContextMenuEventArgs e)
        {
            if (!e.Handled)
            {
                HwndSource hwndSource = PresentationSource.FromVisual(this) as HwndSource;
                if (hwndSource != null)
                {
                    CustomChromeWindow.ShowWindowMenu(hwndSource, this, Mouse.GetPosition(this), base.RenderSize);
                }
                e.Handled = true;
            }
        }
    }
}

Extracted XAML for MainWindowTitleBar:

<mwtb:MainWindowTitleBar Name="MainWindowTitleBar" x:Uid="vs:MainWindowTitleBar_1" Grid.Row="0" Grid.Column="0" Background="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowActiveCaptionBrushKey}}" TextElement.Foreground="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowActiveCaptionTextBrushKey}}">
  <DockPanel x:Uid="DockPanel_2">
    <wcp:SystemMenu Name="SystemMenu" x:Uid="Image_1" Source="{TemplateBinding Window.Icon}" VectorFill="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowActiveIconDefaultBrushKey}}" Width="32" Height="27" Margin="0,0,12,4" Padding="12,7,0,0" DockPanel.Dock="Left" VectorIcon="{Binding Source={x:Static Application.Current}, Path=VectorIcon}" />
    <StackPanel Name="WindowTitleBarButtons" x:Uid="WindowTitleBarButtons" Orientation="Horizontal" DockPanel.Dock="Right">
      <wcp:WindowTitleBarButton Name="MinimizeButton" x:Uid="MinimizeButton" VerticalAlignment="Top" Command="{x:Static vsc:ViewCommands.MinimizeWindow}" BorderBrush="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonActiveBorderBrushKey}}" BorderThickness="1,0,1,1" GlyphForeground="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonActiveGlyphBrushKey}}" HoverBackground="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonHoverActiveBrushKey}}" HoverBorderBrush="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonHoverActiveBorderBrushKey}}" HoverForeground="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonHoverActiveGlyphBrushKey}}" HoverBorderThickness="1,0,1,1" PressedBackground="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonDownBrushKey}}" PressedBorderBrush="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonDownBorderBrushKey}}" PressedForeground="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonDownGlyphBrushKey}}" PressedBorderThickness="1,0,1,1" Padding="0,3,0,0" Width="34" Height="26" AutomationProperties.Name="Minimize" AutomationProperties.AutomationId="Minimize" ToolTip="{x:Static vs:MainWindowResources.WindowMinimizeToolTip}" CommandParameter="{Binding RelativeSource={RelativeSource TemplatedParent}}">
        <Path Name="MinimizeButtonPath" x:Uid="MinimizeButtonPath" Width="9" Height="9" Stretch="None" Data="F1M0,6L0,9 9,9 9,6 0,6z" Fill="{Binding Path=(TextElement.Foreground), RelativeSource={RelativeSource Self}}" />
      </wcp:WindowTitleBarButton>
      <wcp:WindowTitleBarButton Name="MaximizeRestoreButton" x:Uid="MaximizeRestoreButton" VerticalAlignment="Top" Command="{x:Static vsc:ViewCommands.ToggleMaximizeRestoreWindow}" BorderBrush="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonActiveBorderBrushKey}}" BorderThickness="1,0,1,1" GlyphForeground="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonActiveGlyphBrushKey}}" HoverBackground="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonHoverActiveBrushKey}}" HoverBorderBrush="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonHoverActiveBorderBrushKey}}" HoverForeground="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonHoverActiveGlyphBrushKey}}" HoverBorderThickness="1,0,1,1" PressedBackground="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonDownBrushKey}}" PressedBorderBrush="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonDownBorderBrushKey}}" PressedForeground="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonDownGlyphBrushKey}}" PressedBorderThickness="1,0,1,1" Padding="0,3,0,0" Width="34" Height="26" AutomationProperties.Name="Maximize" AutomationProperties.AutomationId="Maximize" ToolTip="{x:Static vs:MainWindowResources.WindowMaximizeToolTip}" CommandParameter="{Binding RelativeSource={RelativeSource TemplatedParent}}">
        <Path Name="MaximizeRestoreButtonPath" x:Uid="MaximizeRestoreButtonPath" Width="9" Height="9" Stretch="Uniform" Data="F1M0,0L0,9 9,9 9,0 0,0 0,3 8,3 8,8 1,8 1,3z" Fill="{Binding Path=(TextElement.Foreground), RelativeSource={RelativeSource Self}}" />
      </wcp:WindowTitleBarButton>
      <wcp:WindowTitleBarButton Name="HideButton" x:Uid="HideButton" VerticalAlignment="Top" Command="{x:Static vsc:ViewCommands.CloseWindow}" BorderBrush="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonActiveBorderBrushKey}}" BorderThickness="1,0,1,1" GlyphForeground="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonActiveGlyphBrushKey}}" HoverBackground="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonHoverActiveBrushKey}}" HoverBorderBrush="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonHoverActiveBorderBrushKey}}" HoverForeground="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonHoverActiveGlyphBrushKey}}" HoverBorderThickness="1,0,1,1" PressedBackground="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonDownBrushKey}}" PressedBorderBrush="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonDownBorderBrushKey}}" PressedForeground="{DynamicResource {x:Static ui:EnvironmentColors.MainWindowButtonDownGlyphBrushKey}}" PressedBorderThickness="1,0,1,1" Padding="0,3,0,0" Width="34" Height="26" AutomationProperties.Name="Close" AutomationProperties.AutomationId="Close" ToolTip="{x:Static vs:MainWindowResources.WindowCloseToolTip}" CommandParameter="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type Window}}}">
        <Path Name="HideButtonPath" x:Uid="HideButtonPath" Width="10" Height="8" Stretch="Uniform" Data="F1M0,0L2,0 5,3 8,0 10,0 6,4 10,8 8,8 5,5 2,8 0,8 4,4 0,0z" Fill="{Binding Path=(TextElement.Foreground), RelativeSource={RelativeSource Self}}" />
      </wcp:WindowTitleBarButton>
    </StackPanel>
    <mwtb:FrameControlContainer Name="PART_TitleBarFrameControlContainer" x:Uid="PART_TitleBarFrameControlContainer" DockPanel.Dock="Right" TextElement.FontSize="{DynamicResource VsFont.EnvironmentFontSize}" TextElement.FontFamily="{DynamicResource VsFont.EnvironmentFontFamily}" Margin="0,0,2,0" DataContext="{Binding FrameControls}" />
    <TextBlock x:Uid="TextBlock_1" Text="{TemplateBinding Window.Title}" TextBlock.FontFamily="{DynamicResource VsFont.CaptionFontFamily}" TextBlock.FontSize="{DynamicResource VsFont.CaptionFontSize}" TextBlock.FontWeight="{DynamicResource VsFont.CaptionFontWeight}" TextTrimming="CharacterEllipsis" VerticalAlignment="Center" Margin="0,7,0,4" />
  </DockPanel>
</mwtb:MainWindowTitleBar>

Stochastic

Is there another StackPanel or DockPanel over it at that area which you haven't set the background for? – Backboard

No, VS has pretty simple visual tree. Updated my question with relevant details. I think maybe its parent element implementing INonClientArea and overriding HitTestCore has something to do with it, but I don't see how it can impact actual hit test so far. – Stochastic

Did you check IsHitTestVisible property value? – Ectomere

Yes, it is true – Stochastic

would it work for you to add a new parent? for example, put the titlebar in a stackpanel x, and subscribe to x mouse events instead? – Commit

I'm pretty sure I'll break something in Visual Studio if I start to change its layout like that. But I've tried to attach MouseEnter and MouseLeave events to VS main window and MouseLeave fires as soon as cursor enters the blue rectangle. – Stochastic

Have you tried using the preview versions of the events? (PreviewMouseEnter/Leave) – Bedroom

@Bedroom there are no PreviewMouseEnter/Leave events. Tried PreviewMouseMove and it doesn't work in blue area. – Stochastic

Answer 1

My original answer was able to detect non-client mouse movements, but not when the mouse left. Based on the OP's finding of needing to use TrackMouseEvent to do this, I have updated my answer to show a fully-functioning example.

As mentioned in the comments, WPF does not handle/wrap non-client area events. I can find no explanation as to why. It is possible though to detect mouse movement (and therefore mouse enter) using a message hook.

From within a VS extension, the hook is initiated with this:

IntPtr vsHandle = System.Diagnostics.Process.GetCurrentProcess().MainWindowHandle;
HwndSource source = HwndSource.FromHwnd(vsHandle);
source.AddHook(new HwndSourceHook(WndProc));

The supporting code is:

private const int WM_NCHITTEST = 0x0084;
private const int WM_NCMOUSEMOVE = 0x00a0;
private const int WM_NCMOUSELEAVE = 0x02a2;

private enum HtResult
{
    HTERROR = (-2),
    HTTRANSPARENT = (-1),
    HTNOWHERE = 0,
    HTCLIENT = 1,
    HTCAPTION = 2,
    HTSYSMENU = 3,
    HTGROWBOX = 4,
    HTSIZE = HTGROWBOX,
    HTMENU = 5,
    HTHSCROLL = 6,
    HTVSCROLL = 7,
    HTMINBUTTON = 8,
    HTMAXBUTTON = 9,
    HTLEFT = 10,
    HTRIGHT = 11,
    HTTOP = 12,
    HTTOPLEFT = 13,
    HTTOPRIGHT = 14,
    HTBOTTOM = 15,
    HTBOTTOMLEFT = 16,
    HTBOTTOMRIGHT = 17,
    HTBORDER = 18,
    HTREDUCE = HTMINBUTTON,
    HTZOOM = HTMAXBUTTON,
    HTSIZEFIRST = HTLEFT,
    HTSIZELAST = HTBOTTOMRIGHT,
    HTOBJECT = 19,
    HTCLOSE = 20,
    HTHELP = 21
}

[Flags]
public enum TMEFlags : uint
{
    TME_CANCEL = 0x80000000,
    TME_HOVER = 0x00000001,
    TME_LEAVE = 0x00000002,
    TME_NONCLIENT = 0x00000010,
    TME_QUERY = 0x40000000
}

[DllImport("user32.dll")]
static extern int TrackMouseEvent(ref TRACKMOUSEEVENT lpEventTrack);

[StructLayout(LayoutKind.Sequential)]
public struct TRACKMOUSEEVENT
{
    public int cbSize;
    public TMEFlags dwFlags;
    public IntPtr hwndTrack;
    public int dwHoverTime;
}

private bool _trackingMouseMove;
private TRACKMOUSEEVENT _tme;

private IntPtr WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
{
    if (msg == WM_NCMOUSEMOVE)
    {
        if (!_trackingMouseMove)
        {
            _tme = new TRACKMOUSEEVENT();
            _tme.hwndTrack = hwnd;
            _tme.cbSize = Marshal.SizeOf(typeof(TRACKMOUSEEVENT));
            _tme.dwFlags = TMEFlags.TME_NONCLIENT | TMEFlags.TME_LEAVE;
            int success = TrackMouseEvent(ref _tme);
            _trackingMouseMove = (success != 0);
        }

        var hitTestResult = (HtResult)wParam;

        if ((hitTestResult == HtResult.HTSYSMENU) || (hitTestResult == HtResult.HTCAPTION))
        {
            // Raise event here
            System.Diagnostics.Debug.WriteLine("Mouse over title bar");
        }
    }
    else if (msg == WM_NCMOUSELEAVE)
    {
        _trackingMouseMove = false;
        System.Diagnostics.Debug.WriteLine("Mouse left the title bar");
    }

    return IntPtr.Zero;
}

Elconin

Not sure why this received downvotes, it seems like a correct solution although it uses P/Invoke. Sadly I've badly formulated my question and forgot to mention that my initial problem is subscribing to MouseEnter and MouseLeave events which work incorrectly (IsMouseOver is a symptom that is easier to debug) and your solution doesn't solve my initial problem. I've edited my question to reflect that. – Stochastic

FWIW, before posting this, I used WPF Snoop to check which mouse events the MainWindowTitleBar responds to. It doesn't respond to any mouse events (or any other event that I could see), so it may not be possible without P/Invoke. I look forward to seeing what others may find. – Elconin

But how it's technically possible to have a WPF Panel that does not respond to mouse events? – Stochastic

MainWindowTitleBar falls within the window's non-client area, whose events are apparently not handled/wrapped by WPF, though I can't find an explanation of why. From my research, it should be possible to hook these events using HwndSource.AddHook. I'll try to come up with an example of this this evening. – Elconin

Another gotcha is transparency. If the background of your control is transparent it will only trigger when over something with Opacity > 0. (bitten me so many times.) – Abbottson

@Abbottson that's why I've explicitly set background to red – Stochastic

@Elconin looks like you are right. I've tried to attach MouseEnter and MouseLeave events to VS main window and MouseLeave fires as soon as cursor enters the blue rectangle (assuming it was inside main window before that) – Stochastic

WM_NCMOUSELEAVE is not fired because you need to call TrackMouseEvent (P/Invoke) to start tracking for it. Now my solution is complete. This looks like the easiest possible way to do it, thanks! – Stochastic

By the way I used this to create an extension that switches main menu to auto-hide in Visual Studio – Stochastic