Could you share the information as Shibani requested.
Assuming you are using Pega 7.1.8, from the screenshot, I understand that you have set the tab border( as say style: solid ; color: blue). If yes, is it possible not to set the border styles for the tab control.
Also, are you using/set responsiveness feature on the column repeat control? This is to understand when you say the border is not expanded dynamically. I see that this feature is not available under tab control's skin.