@MoonKnight answer is the only answer that can draw decent border at windows 11.

so took it, instead of adding BorderedTextbox on desinger.

will Wrap your textbox at the form_load, so you'll be still referencing. the same textbox. same events etc.

atDesigner:

while Running:

how to use

private void Form1_Load(object sender, EventArgs e) 
{
   //this method will preserve textbox itself, events etc.
   // but it will wrap textbox inside BorderedPanel.
   var borderedPanel = TextBox_Helper.MakeTextbox_CustomBorderColor(textbox1); 
         
}

required Class:

  public static class TextBox_Helper
  {
      /// <summary>
      /// wrap your exisiting textbox, into a panel with customColor Roudned borders
      /// </summary>
      /// <returns>retunrs  BorderedTextbox ,  aka The Panel That Wraps The your Exisiting Textbox</returns>
      public static BorderedTextBox MakeTextbox_CustomBorderColor(this TextBox tbx) 
      {
          //backup previous TBX status
          var tbxOriginalParent =  tbx.Parent;
          var loc = tbx.Location;
          var size = tbx.Size;
          var anchor = tbx.Anchor;
          var dock = tbx.Dock;

          //restore them to Panel
          var bordered_tbx = new BorderedTextBox(tbx);
          bordered_tbx.Location = loc;
          bordered_tbx.Size= size;
          bordered_tbx.Height= size.Height+5;
          bordered_tbx.Anchor = anchor;
          bordered_tbx.Dock = dock;
          tbxOriginalParent.Controls.Add(bordered_tbx);

          return bordered_tbx;
      }
  }

  // https://stackoverflow.com/questions/17466067/change-border-color-in-textbox-c-sharp/39420512#39420512
  public class BorderedTextBox : Panel
  {
      private TextBox textBox;
      private bool focusedAlways = false;
      private Color normalBorderColor = Color.LightGray;
      private Color focusedBorderColor = Color.FromArgb(0,00,225);
      //private Color focusedBorderColor = Color.FromArgb(86,156,214);

     
      public TextBox TextBox
      {
          get { return textBox; }
          //set { textBox = value; }
      }
      public bool FocusedAlways
      {
          get { return focusedAlways; }
          set { focusedAlways = value; }
      }

      public BorderedTextBox(TextBox tbx = null)
      {
          this.DoubleBuffered = true;
          this.Padding = new Padding(2);

          if (tbx == null)
              textBox = new TextBox();
          else
              textBox = tbx;

          this.TextBox.AutoSize = false;
          this.TextBox.BorderStyle = BorderStyle.None;
          this.TextBox.Dock = DockStyle.Fill;
          this.TextBox.Enter += TextBox_Refresh;
          this.TextBox.Leave += TextBox_Refresh;
          this.TextBox.Resize += TextBox_Refresh;
          this.Controls.Add(this.TextBox);

          this.textBox.FontChanged += TextBox_FontChanged;
          RefreshHeight(textBox);

          //debug helper
          //this.TextBox.BorderStyle = BorderStyle.FixedSingle;
          this.BackColor = Color.Red;

      }

      private void TextBox_FontChanged(object sender, EventArgs e)
      {
          RefreshHeight(textBox);
          this.Height = textBox.Height+5;
      }

      private void TextBox_Refresh(object sender, EventArgs e) => this.Invalidate();

      protected override void OnPaint(PaintEventArgs e)
      {
          e.Graphics.Clear(SystemColors.Window);

          using (Pen borderPen = new Pen(this.TextBox.Focused || focusedAlways ?
              focusedBorderColor : normalBorderColor))
          {
              //e.Graphics.DrawRectangle(borderPen,
              e.Graphics.DrawRoundRectangle(borderPen,
                  new Rectangle(0, 0, this.ClientSize.Width - 1, this.ClientSize.Height - 1)
                  ,5);
          }
          base.OnPaint(e);
      }


      /// <summary>
      /// fix Textbox.borderNone bottom gets clipped issue
      /// </summary>
      /// <param name="textbox"></param>
      static void RefreshHeight(TextBox textbox)
      {
          return;

          textbox.Multiline = true;
          Size s = TextRenderer.MeasureText("AĞÜüğGgpPa", textbox.Font, Size.Empty, TextFormatFlags.TextBoxControl);
          textbox.MinimumSize = new Size(0, s.Height + 1 + 3);
          textbox.Multiline = false;
      }


  }