[macOS] context menu doesn't stay open in a window that doesn't have focus

Steps to reproduce:

• run the sample attached
• make the app window lose focus, for example by clicking on the desktop
• right click on the window
  EXPECTED: a context menu opens, and stays open while the mouse cursor is moved around the screen.
  ACTUAL: a context menu opens, but it closes when you move the mouse cursor away from the context menu.

The buggy behavior reproduces with Qt Creator 4.6.1 as well.
For comparison test with 'Finder', for example.