[REG] QTabWidget is rendered incorrectly after dragging if it is central widget and QMainWindow::unifiedTitleAndToolBarOnMac is set to true

1. Run attached project
2. Drag tab to another position

The tab is rendered with artefacts.

Without wnd.setUnifiedTitleAndToolBarOnMac( true ); line it works as expected.

With Qt 5.13.1 it works better - the tab background is transparent while dragging, but after dragging it is rendered correctly.