Run the HelloTabPane toy. Create a few new tabs. Switch the position of the tab pane header so that it is on the left side of the window.
Now switch to some of the new tabs (or some Tab 6, 7, or whatever at the end (or rather, the bottom) of the list) and delete it by clicking x in its header.
Watch the header corresponding to the Tab 1 at the top of the header. Once you remove one or several tabs from the bottom, you no longer see a part of the Tab 1 header. It is rendered as a solid white rectangle. After you switch to the Tab 1, things get back to normal until next time you remove a tab from the bottom of the list.