[Reg 5.8->5.9] Visual apperance of QTabBar with expanding tabs has changed

The visual appearance/behavior of expanding tabs have changed. In older versions the tabs did not take all the space as they do from 5.9 onwards.

Steps to reproduce:

• Build the sample attached using Qt 5.8 (or older), and run the app.
• Build the sample using Qt 5.9, and run the app.
• Observe the difference in appearance between the Qt versions.
  EXPEXTED: There should not be surprising and undocumented changes to the appearance/behavior.
  ACTUAL: The appearance/behavior has changed.