[macOS] Semi-transparent widget won't be rendered correctly when resizing quickly on macOS 10.13 and 10.12

Rendering incorrectly (rendering line or shadow on transparent area) when resizing from big to smaller quickly by grabbing semi-transparent gray area in bottom right in sample code on macOS 10.13 and 10.12. It works normally on macOS 10.14 or using Qt 5.9.