Qt5/QML program does not use compositing anymore
I'm running my application on Kubuntu 17.04, X11. I use Qt 5.9.2.
My application uses QQuickWindow to draw panels and docks. The window itself is fully transparent, but loads a QML file that uses semi-transparent elements. It worked just fine for years.
But recently (after some systems management), my application does not use compositing anymore. In other words: all my QQuickWindows have opaque black backgrounds, ruining the look of my program.
I use Compton for compositing. Interestingly, my application seems to be the only one with issues - compositing seems to work fine for other applications. I tried resetting compton's configuration, and even reinstalling it, but the problem remains. I suspect the problem started to occur when I removed a bunch of packages from my system I believed I didn't need at all, but I'm not sure, and I haven't got a clue what package could cause this. I tried rolling back all the changes I made, but the problem persists.
Can anyone help? I'm supposed to demonstrate my program to some people in two days, and I'd hate to have to show it to them like this.
Does it also happen if you use your distribution provided Qt ?