Hi Martin,
There are some ways to achieve this with CSS.
One of which is making the button position absolute and control its position without needing to be "outside" of the tab.
You can see for instance this example, and adjust the right and top properties to fit your purpose:
I would have a container enclosing the tabs widget and the button. The container would be the element with position relative in the example and your button would be the one absolute.
Hope it helps.
Kind Regards,
João