This is not my control I downloaded it from this page :

CodePudding user response：

At the bottom of the OnPaint override method in the MACTrackBar.cs file, comment out these lines:

// Draws a focus rectangle
// if(this.Focused)
//   ControlPaint.DrawFocusRectangle(e.Graphics, 
//                                   Rectangle.Inflate(this.ClientRectangle, -2, -2));