I have found a new behavior in our Qt application since the High Sierra update.

It seems that when starting up and setting a QuickWindow to fullscreen, the window will not be properly fullscreen. The topbar still appears on top of the window and it's not really having the focus.

To isolate this and make sure it wasn't part of our app i used the minimal webengine sample and just added a showFullScreen call on the onvisibilitychanged event.

it behaves exactly the same (see screenshot below)

As you can see running this from QtCreator will keep the QtCreator menubar and actually QtCreator application is keeping the focus.

I made a sample from the minimal browser sample with these modifications so that it can be reproduced easily. You will fin it here https://github.com/LongChair/qtquick

I also tested that if i run the application normally and add a keypress event in which i would do showNormal or showFullscreen everything seems to behave normally and I don't have that issue. So it seems something is going wrong only when setting Fullscreen mode on startup.

Also this issue happens both qt Qt 5.9.X and recent Qt 5.11.0. beta3. 

I'm available for further testing, this  issue is really annoying for our application.