(macOS) using Cocoa WebView with QMacCocoaViewContainer and QWidget turns blank on focus loss

On macOS 10.14 and 10.15, we counter a GUI problem of blank view when using together a cocoa WebView/QMacCocoaViewContainer/QWidget to render an HTML page.

(Please see the attached video clip)

Some important information:

• Under the hood, the WebView is part of a QMacCocoaViewContainer, and eventually attached to a QWidget.
• the web page is rendered properly when showing the QWidget. When clicking outside the widget to cause focus los, the entire view becomes blank. As soon as the widget gets focus (by clicking on it), the web page is fully rendered again.
• This blank view issue doesn't happen when using Qt5.5. Also it doesn't happen on MacOS 10.13.

It seems to be a problem with window painting or conflicts between Qt5.12.2 and Cocoa WebView