您需要将窗体的Location属性设置为新位置，而不是设置Cursor.Position.X和Cursor.Position.Y属性。这将防止窗体在WindowState设置为Normal时传送到左上角。
更新后的代码应该如下所示：

private void TopBarNav_MouseDown(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left)
    {
        isDragging = true;
        offset = e.Location;
    }
}
private void TopBarNav_MouseMove(object sender, MouseEventArgs e)
{
    if (isDragging)
    {
        this.WindowState = FormWindowState.Normal;
        ExpandButton.IconChar = IconChar.Expand;
        Region = System.Drawing.Region.FromHrgn(CreateRoundRectRgn(0, 0, Width, Height, 20, 20));
        Point newLocation = new Point(
        this.Location.X + e.X - offset.X,
        this.Location.Y + e.Y - offset.Y
    );

    this.Location = newLocation;
    }
}

private void TopBarNav_MouseUp(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left)
    {
        isDragging = false;
    }
}

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