Thank you for you pull request. I will review it within the next days.

I tested the pull request on Kubuntu 18.04 and it does not work properly. Maximizing a dock widget works but resetting the previous geometry from maximized state does not work. The value currentScreen is always nullptr if the window is in maximized state. The following code works better.

QRect widgetRect = this->geometry();
    widgetRect.adjust(1,1,-1, -1);
    QScreen* currentScreen = nullptr;
    for (auto screen : QGuiApplication::screens())
    {
        auto screenRect = screen->geometry();
        if (screenRect.contains(widgetRect, true))
        {
            currentScreen = screen;
        }
    }

Another problem is, that on windows as soon as the maximized window is dragged via titlebar, the maximize button immediatelly changes its state to maximize functionality. This does not happen with your implementation.

Therefore I cannot accept the pull request as is.

Thanks for your advice, I'll fix it

So, did this PR die ?
It's a feature i, and probably a lot of other people, would love to have.

I would be happy to test and poke the code a bit if required.

You can checkout the code of the pull request and play around with it.