wm_nchittest/httopleft in WPF

Hello,

I am porting a Windows Forms app to WPF but I'm struggling with capturing
nchittest message. Basically, the following code in my Windows Forms app
resizes the window in the topleft direction.

private const int HTTOPLEFT = 13;
private const int WM_NCHITTEST = 0x84;
protected override void WndProc(ref System.Windows.Forms.Message m)
{
if (m.Msg == WM_NCHITTEST)
{
m.Result = new System.IntPtr(HTTOPLEFT);
return;
}
base.WndProc(ref m);
}

In WPF, I've mimicked this behavior by capturing the mouse, then setting the
top/left and width/height properties of the window while moving the mouse.
Unfortunately this hack is not desirable. What happens is that the window
moves then resizes several times while moving the mouse. It does not look
as smooth as using the actual size grip.

Any suggestion is appreciated.

Reggie