protected override void OnMouseDown(System.Windows.Forms.MouseEventArgs e)
{
isMouseDown = true;
base.OnMouseDown(e);
appCanvas.Resize(e.X, -e.Y);
IntPtr hwnd = this.Handle;
IntPtr hdc = WinNative.GetDC(hwnd);
//canvas paint, clear bg
appCanvas.RenderTo(hdc);
WinNative.ReleaseDC(hwnd, hdc);
}