When the tab widths consume more space than available, the QTabBar usually shows left/right buttons (if enabled) to scroll to the previous/next tab. 

As soon as the last tab is hidden the buttons cease to work.

Workaround with flaws: move the last visible tab to the last tab position. But that has the drawback that the tabs are getting shuffled by time and it's not easy to restore the tab position after some tabs have been opened and closed in the meantime.

This may overlap with QTBUG-102020