Form design via Region property in winform getting removed - winforms

Question

I have customized winform-design using region property as follows,
Region = System.Drawing.Region.FromHrgn(CreateRoundRectRgn(0, 0, varPassedInConstructor * 9, Height, 10, 10));
And here calling winform through following code in a new thread
new Thread(new ThreadStart(() => {
toast toast = new toast(message);
toast.Show(nativeWindow);
toast.Refresh();
Thread.Sleep(3000);
while (toast.Opacity > 0)
{
toast.Opacity -= 0.04;
Thread.Sleep(100);
}
toast.Close();
toast.Dispose();
})).Start();
Everything goes well, form is displayed properly initially, but before closing all of sudden, changes applied via Region gets disappeared and form appears like the one that is at design time.
Image one, When initially form displayed,
Image two, just before form is getting closed,
I tried lots of diff thing, I am not getting what exactly problem is, so all help will be appreciated.

Answer 1

Finally, I got the fix, instead of using CreateRoundRectRgn from GDI32 used a GraphicsPath approach as follows,
private void SetRegion()
{
var GP = RoundedRect(this.ClientRectangle, 5);
this.Region = new Region(GP);
}
And here is code for RoundRect function (Credit goes to https://stackoverflow.com/a/33853557/3531672),
public static GraphicsPath RoundedRect(Rectangle bounds, int radius)
{
int diameter = radius * 2;
Size size = new Size(diameter, diameter);
Rectangle arc = new Rectangle(bounds.Location, size);
GraphicsPath path = new GraphicsPath();
if (radius == 0)
{
path.AddRectangle(bounds);
return path;
}
// top left arc
path.AddArc(arc, 180, 90);
// top right arc
arc.X = bounds.Right - diameter;
path.AddArc(arc, 270, 90);
// bottom right arc
arc.Y = bounds.Bottom - diameter;
path.AddArc(arc, 0, 90);
// bottom left arc
arc.X = bounds.Left;
path.AddArc(arc, 90, 90);
path.CloseFigure();
return path;
}
then in constructor simply had set size of form itself and called SetRegion defined above,
this.Width = toastMessage.Length * 9;
SetRegion();
Please note additionally, I would recommend to override OnSizeChanged and simply call SetRegion in it.