On Mac OS X, the System Preferences can be changed to allow a user to use the tab key to move focus between all controls. See:

System Preferences-> Keyboard & Mouse-> Keyboard Shortcuts

When the focus changes, a halo is drawn around the control currently in focus.

If this control is a QTabBar, the halo is not drawn correctly. It appears to be clipped at the top and bottom thus the halo only appears to the side of the control whereas the halo should surround the entire control as it does in native applications with tab bars.