I can see that in MainWindow.h and MainWindow.cpp this pattern is doing more harm than good. However, it seems to make sense from client-code perspective. Hence, in these two specific cases (this line and line 437), you could create methods such as showTabBar and hideTabBar.

That's not really possible currently as the tabbar is unrelated to the window, what to do in the case of two tabbars, etc.
in a following patch I plan to ditch the tabbar management code and replace with something more sane.