[macOS] Regression from 5.9: QMacCocoaViewContainer broken in macOS 10.13 & 10.12 (Sierra/High Sierra)

If you compile example project against 5.10/5.11/5.12/5.13/5.14, then run it in macOS 10.13 (High Sierra or 10.12 Sierra) you get 3 problems (well seen on the BrokenCocoa.mp4):
1. Segmented controls doesn't appear until you click them (in a real app, they also blink and hide/show)
2. All cocoa controls disappear when window looses focus (click on the wifi icon)
3. Spinbox doesn't change when you drag slider and it freezes
4. When you resize the last 'Bug' column you get lots of rendering glitches.

• If you compile this project against 5.6.3/5.9.8, it's totally fine.
• It's always fine in 10.14 & 10.15

Screenshots are also attached. We do think it's a serious issue. Basically our app doesn't work on macOS 10.12/10.13 while these OSes are officially supported by Qt.