How can I achieve smooth WPF window dragging with stylus input?

I had a similar problem, except it was Windows Forms rather than WPF, and a mouse rather than a stylus. I expect the solution should work for WPF and stylus as well – the idea is to translate the click/touch in the client area to a ditto in the non-client area, thus fooling Windows into thinking you grabbed the title bar. Windows then does all the rest for you.

csharp
public const int WM_NCLBUTTONDOWN = 0xA1;
public const int HT_CAPTION = 0x2;

[DllImport("user32.dll")]
public static extern bool ReleaseCapture();

[DllImport("user32.dll")]
public static extern int SendMessage(IntPtr hWnd, int Msg, int wParam, int lParam);

public void Handle_MouseDown(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left)
    {
        ReleaseCapture();
        SendMessage(Application.OpenForms[0].Handle, WM_NCLBUTTONDOWN, HT_CAPTION, 0);
    }
}

(You obviously won't have any Application.OpenForms; off the top of my head I'm not sure what the equivalent in WPF would be – Window.Handle of some sort.)

Update (WPF instead of WinForms)

I rewrote this for WPF and ran the result on a Surface. Works with stylus, touch and mouse, as stylus and touch messages are forwarded as mouse messages if not handled. (Handling stylus and touch events directly is probably an alternative, but this certainly works.)

XAML

xaml
<Window
  x:Class="Stylus.MainWindow"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:local="clr-namespace:Stylus">
  <Grid
    MouseDown="Handle_MouseDown"
    Background="Transparent" />
</Window>

Code

csharp
namespace Stylus;

using System.Runtime.InteropServices;
using System.Windows;
using System.Windows.Input;
using System.Windows.Interop;

public partial class MainWindow : Window
{
    public const int WM_NCLBUTTONDOWN = 0xA1;
    public const int HT_CAPTION = 0x2;

    [DllImport("user32.dll")]
    public static extern bool ReleaseCapture();

    [DllImport("user32.dll")]
    public static extern int SendMessage(IntPtr hWnd, int Msg, int wParam, int lParam);

    public MainWindow() => InitializeComponent();

    private void Handle_MouseDown(object sender, MouseButtonEventArgs e)
    {
        ReleaseCapture();
        SendMessage(new WindowInteropHelper(this).Handle, WM_NCLBUTTONDOWN, HT_CAPTION, 0);
    }
}

You should probably not do anything in the StylusMove event handler when the Stylus is not in the down state.

Use a Nullable<Point> for pressPosition and only set it on StylusDown. In StylusMove, check whether it has a value, and reset it to null in StylusUp. Works smoothly for me - with a finger on my touchscreen.

CSHARPCopied
private Point? pressPosition;

private void Window_PreviewStylusDown(object sender, StylusDownEventArgs e)
{
    if (CaptureStylus())
    {
        pressPosition = e.GetPosition(this);
    }
}

private void Window_PreviewStylusMove(object sender, StylusEventArgs e)
{
    if (pressPosition.HasValue)
    {
        var currentPosition = e.GetPosition(this);
        Left += currentPosition.X - pressPosition.Value.X;
        Top += currentPosition.Y - pressPosition.Value.Y;
    }
}

private void Window_PreviewStylusUp(object sender, StylusEventArgs e)
{
    ReleaseStylusCapture();
    pressPosition = null;
}