[MAC][5.6] Setting window flags (window type) is broken

The reported bug is fixed in Qt 5.10!

Newer versions of macOS support Split View - a way to see windows side-by-side in Full Screen. This makes the option to be able to switch window type on the fly more important then ever - the user might want to have a floating window on top of Full Screen application OR might want the window on the side. The first is served by making the window a Qt::Tool, the latter - by making a normal Qt::Window. This transition is however broken in 5.6, particularly moving from 'tool' to 'window'.

Supplied is a test case. Compile and run it. 

Double click on the window to create a new (tool) window. Double-clicking on the new red window will toggle it b/w Qt::Tool and Qt::Window. You will see how the window is in broken state when it is first double-clicked and turned into a Qt::Window.

If you change the code to init to a Qt::Window, on first double-click, it successfully transitions to a Qt::Tool but will fail to get back to a normal, non-broken Qt::Window though it will be "a bit better" (still no Full Screen button, possibly other issues).

All this works correctly on Qt 5.10, but with a deployment target of macOS 10.10 (2014!), using it is a no-go for a lot of people! If this is is not patched, please provide pointers for fixing it ourselves. Thanks.