Provide QWidget::setScreen

We want to get rid of QDesktopWidget, so we also need to provide an alternative to creating toplevel windows with a QDesktop(Screen)Widget as the parent in order to have them created on the correct screen.

Assuming a virtual desktop with two screens, 0 to the left and 1 to the right. Code like:

QWidget *topLevel = new QWidget;
topLevel->setScreen(QGuiApplication::screens().at(0));
topLevel->show();

This should make the toplevel window open up on the presumably left-most screen, no matter if

• screen 0 is part of a virtual desktop group
• there are other application windows already open on another screen
• the mouse cursor is on the second screen

Maximizing the window should maximize it on that screen; restoring it should bring it back to the previous position on that screen. The widget should have the devicePixelRatio of screen 0.

While the toplevel widget is visible, calling

toplevel->setScreen(QGuiApplication::screens().at(1));

should NOT move the visible window to the second screen. However, maximizing the window should now fill the second screen; restoring it from there should restore it back to the previous geometry.

Assuming a multi-head setup, ie. no virtual desktop (and only relevant on X11 or Wayland), both calls to QWidget::setScreen should result in the window being placed on the second screen.

Qt has QWindow::setScreen, but QWidget typically doesn't return a valid QWindow from windowHandle() until it's "too late", ie until the widget is becoming visible.