When I call CaptureMouse() in response to a MouseDown from the middle mouse button, it will capture and then release the mouse.

Huh?

I've tried using Preview events, setting Handled=true, doesn't make a difference. Am I not understanding mouse capture in WPF?

Here's some minimal sample code that reproduces the problem.

// TestListBox.cs

using System.Diagnostics;
using System.Windows.Controls;

namespace Local
{
    public class TestListBox : ListBox
    {
        public TestListBox()
        {
            MouseDown += (_, e) =>
            {
                Debug.WriteLine("+MouseDown");
                Debug.WriteLine(" Capture: " + CaptureMouse());
                Debug.WriteLine("-MouseDown");
            };

            MouseUp += (_, e) =>
            {
                Debug.WriteLine("+MouseUp");
                ReleaseMouseCapture();
                Debug.WriteLine("-MouseUp");
            };

            GotMouseCapture += (_, e) => Debug.WriteLine("GotMouseCapture");
            LostMouseCapture += (_, e) => Debug.WriteLine("LostMouseCapture");
        }
    }
}

Generating a default WPF app that has this for its main window will use the test class:

<Window x:Class="Local.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:Local"
        Title="MainWindow" Height="350" Width="525">
    <local:TestListBox>
        <ListBoxItem>1</ListBoxItem>
        <ListBoxItem>2</ListBoxItem>
        <ListBoxItem>3</ListBoxItem>
        <ListBoxItem>4</ListBoxItem>
    </local:TestListBox>
</Window>

Upon clicking the middle button down, I get this output:

+MouseDown
GotMouseCapture
LostMouseCapture
 Capture: True
-MouseDown

So I'm calling CaptureMouse, which in turn grabs and then releases capture, yet returns true that capture was successfully acquired.

What's going on here? Is it possible that this is something with my Logitech mouse driver doing something goofy, trying to initiate 'ultrascroll' or something?