I found out yesterday night this issue occured when "Summer" Theme is used.
I sent a support ticket along with a project to reproduce it.
This issue was initiated on the Tabcontrol thread.
I just received a message stating that it has been resolved and will be included in the next internal build.
To reproduce set the Summer theme in your App constructor as follows:
this.Startup += this.Application_Startup;
this.Exit += this.Application_Exit;
this.UnhandledException += this.Application_UnhandledException;
StyleManager.ApplicationTheme = ThemeManager.FromName(