Since WF 2.6.3b16 widget borders have display issues. Some border are thick. Some are thin. The space between borders is gone: tabs & borders overlap. Widgets in columns have borders that collide. Widgets can have 3 sides with thick border and one side with thin border. Or a side with extra thick border.
I also had this problem before, and have it still in newest version. See an example of tabbed and non-tabbed widget in my site: In custom style: in default style:
Yes, that is similar to mine. On my style widgets collide as well. The top border touches the bottom border of the widget above. I did not have this problem in the previous versions.
Hmm, the tabs border has been there for quite a while. However, in this screenshot There should be a line below the tab headers. Like this: Anyway, if you don't like the tab body border, you can customize your style property (Tabs panes container): I'm not sure about @alfa1's spacing issue though. There should be spacing between widget as mandated by XenForo '.section' css rule (in default style, the spacing is 10px).