Given the following RandR setup:

• Screen 0 - 1680x1050, right of
• Screen 1 - 1280x1024,

QDesktopWidget::screenNumber always returns screen 1 during startup,
even if it is started on screen 0.

This behavior already existed with KDE 4, and it persists with Plasma 5.5.3 and Qt 5.5.1. The reason I noticed it is that the KDE feature to restore window sizes on startup seems to depend on that screenNumber call during application startup as well - and it does not work screen 0.

In the attached test program, the following method is called both in the main window constructor and manually through a button:

void MainWindow::logCurrentScreenInfo() {
    const int scnum = QApplication::desktop()->screenNumber(window());
    QRect desk = QApplication::desktop()->screenGeometry(scnum);

    qDebug() << "Screen #" << scnum
             << ",\nDesktop=" << desk
             << ",\nWindow=" << window()->pos() << " " << window()->geometry();
}

MainWindow constructor:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    qDebug() << "Screen info during main window construction:\n";
    logCurrentScreenInfo();
}

When I start the attached project in QtCreator on screen #0, the output looks like the following:

Screen info during main window construction:

Screen # 1 ,
Desktop= QRect(0,0 1280x1024) ,
Window= QPoint(0,0)   QRect(0,0 258x96)

Screen info on button click:

Screen # 0 ,
Desktop= QRect(1280,0 1680x1050) ,
Window= QPoint(2625,0)   QRect(2629,29 258x96)

When I launch the program on screen #1, the output is as expected (screen 1 is returned from both calls).

I don't know if the Qt behavior is expected and the issue is with KDE (or KWin), or if this behaviour is indeed a bug that should be fixed.