Winform remain unexpected painted color on other screens after snap to main screen's corner

Question

I am creating a simple borderless winform witch remain window snapping feature.

But when I snap winform to the corner of some screen.
It still remain some unexpected painted color on other screens as in example Picture1.
I have attached Form1 in c# code here for your reviewing.

Your help or any suggesting would be appreciated if I am wrong or missing somethings.
If another way to archive this, please do not hesitate to let me know.

Thank you in advance.

Picture1:

namespace Test.FormApp
{
    public partial class Form1 : Form
    {
        private const int WM_NCCALCSIZE = 0x0083;
        private const int WM_NCHITTEST = 0x0084;
        private const int HTCAPTION = 2;

        public Form1()
        {
            InitializeComponent();

            this.SetStyle(ControlStyles.ResizeRedraw, true);
            this.DoubleBuffered = true;
        }

        protected override void WndProc(ref Message m)
        {
            switch (m.Msg)
            {
                case WM_NCCALCSIZE:
                    if (m.WParam != IntPtr.Zero)
                    {
                        return; // Leave out when WParam != 0 to eliminate Form1 borders but remain Window snapping feature.
                    }
                    break;
                case WM_NCHITTEST:
                    if (m.Result == IntPtr.Zero)
                    {
                        m.Result = (IntPtr)HTCAPTION; // Just to move Form1 to demonstrate the problem in Picture1.
                        return;
                    }
                    break;
            }

            base.WndProc(ref m);
        }

        protected override void OnPaint(PaintEventArgs e)
        {
            base.OnPaint(e);

            using (var gp = new GraphicsPath())
            {
                // Painting Form1 rounded rectangle borders to investigate the problem in Picture1.
                var rect = new Rectangle(0, 0, this.Width, this.Height);
                gp.AddRectangle(rect);
                rect.Inflate(-20, -20);
                gp.AddRectangle(rect);
                e.Graphics.FillPath(Brushes.Green, gp);
            }
        }
    }

    partial class Form1
    {
        /// <summary>
        /// Required designer variable.
        /// </summary>
        private System.ComponentModel.IContainer components = null;

        /// <summary>
        /// Clean up any resources being used.
        /// </summary>
        /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        #region Windows Form Designer generated code

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent()
        {
            this.SuspendLayout();
            // 
            // Form1
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(682, 456);
            this.Name = "Form1";
            this.Text = "Form1";
            this.ResumeLayout(false);

        }

        #endregion
    }
}