Resizable Form with BorderStyle None and Child with Dock Fill

My goal is to create window (using WindowsForms) that is resizable like normal Win10 windows but has no titlebar, so I can draw it myself (like UWP apps).

I finally found a really good working anwser by modifing this code: https://stackoverflow.com/a/29788300/15213858

What I have now:

public class MyForm : Form
{
    //Window Messages
    public const uint WM_NCPAINT = 0x85;
    public const uint WM_NCCALCSIZE = 0x83;
    public const uint WM_NCHITTEST = 0x84;

    //RECT Structure
    [StructLayout(LayoutKind.Sequential)]
    public struct RECT
    {
        public int left, top, right, bottom;
    }

    //WINDOWPOS Structure
    [StructLayout(LayoutKind.Sequential)]
    public struct WINDOWPOS
    {
        public IntPtr hwnd;
        public IntPtr hwndinsertafter;
        public int x, y, cx, cy;
        public int flags;
    }

    //NCCALCSIZE_PARAMS Structure
    [StructLayout(LayoutKind.Sequential)]
    public struct NCCALCSIZE_PARAMS
    {
        public RECT rgrc0, rgrc1, rgrc2;
        public WINDOWPOS lppos;
    }

    //Window Procedure Hook
    protected override void WndProc(ref Message m)
    {
        //Don't style window in designer...
        if (DesignMode)
            base.WndProc(ref m);

        //Handle Message
        switch ((uint)m.Msg)
        {
            case WM_NCCALCSIZE: WmNCCalcSize(ref m); break;
            default: base.WndProc(ref m); break;
        }
    }

    //WM_NCCALCSIZE
    private void WmNCCalcSize(ref Message m)
    {

        //Check WPARAM
        if (m.WParam != IntPtr.Zero)    //TRUE
        {
            //When TRUE, LPARAM Points to a NCCALCSIZE_PARAMS structure
            var nccsp = (NCCALCSIZE_PARAMS)Marshal.PtrToStructure(m.LParam, typeof(NCCALCSIZE_PARAMS));

            //We're adjusting the size of the client area here. Right now, the client area is the whole form.
            //Adding to the Top, Bottom, Left, and Right will size the client area.
            nccsp.rgrc0.top += 0;      //30-pixel top border
            nccsp.rgrc0.bottom -= 8;    //4-pixel bottom (resize) border
            nccsp.rgrc0.left += 8;      //4-pixel left (resize) border
            nccsp.rgrc0.right -= 8;     //4-pixel right (resize) border

            //Set the structure back into memory
            Marshal.StructureToPtr(nccsp, m.LParam, true);
        }
        else    //FALSE
        {
            //When FALSE, LPARAM Points to a RECT structure
            var clnRect = (RECT)Marshal.PtrToStructure(m.LParam, typeof(RECT));

            //Like before, we're adjusting the rectangle...
            //Adding to the Top, Bottom, Left, and Right will size the client area.
            clnRect.top += 0;      //30-pixel top border
            clnRect.bottom -= 8;    //4-pixel bottom (resize) border
            clnRect.left += 8;      //4-pixel left (resize) border
            clnRect.right -= 8;     //4-pixel right (resize) border

            //Set the structure back into memory
            Marshal.StructureToPtr(clnRect, m.LParam, true);
        }

        //Return Zero
        m.Result = IntPtr.Zero;
    }
}