Hi Rajani,
We were able to see the problem and we have logged a bug report #2747 for this.
Defect #2747 - XPTaskBarBox Header border is not visible when focus changes from one box to other.
As a workaround, you can set the FocusOnClick to false and set the colors of header in the Enter and Leave events to differentiate the focussed box. Note: This will work only if the ThemesEnabled property is not set. Please refer the attached sample.
Forum_49708.zip
Thanks for choosing Syncfusion products.
Regards,
K.Saradha Devi.