The pictures examples show how Qt 5.15 Beta4 has inconsistent tab title with (on the edges) on QTabWidget, depending on which tab is selected. Qt 5.12.6 is shown for reference without these problems.

 Qt 5.15 beta4 clearly looks bad.

Building on macos 10.15.4 with Xcode 10.3.

I should also add that Qt 5.15 layout behaviour generally - e.g. default layout spacing and control edge widths - is less refined and less consistent that previous Qt versions. I don't know why things had to be changed, but Qt 5.15 clearly looks unfinished (compare to 5.12) when I compile all my apps with it.