In case anyone needs a more detailed explanation the need for getTabAt(tabN.numChildren-1).
We cannot access tabN.tabBar because of it's protected attribute so we cannot directly get the tabBar's width. That is why we are getting the last-tab's button instance x + witdh , It's the exact width of the tabBar. The x of the (last-tab's button instance) is the sum of all preceding tab button instances + any style the tabnavigator might set. Again, :) not trying to offend anyone, just that not everyone will understand exactly what getTabAt(tabN.numChildren-1) means.
