In my opinion, I think the highlight on the tabs should be on the left side. It flows better, because, like most other tab highlights in the UI, they flow top => down or left => right (at least for LTR languages)
Additionally, the highlight bar butting up against a single pixel gray vertical line is jarring and distracting depending on the content in the center panel.
When on the left, it still highlights the tab, doesn't look like a scrollbar (as per the comments), and doesn't clash with the vertical line content separator. The only difference is it pushes the text in the tab in by the width of the border, so the words don't exactly line up.
Current:
Tweaked:
