Bring window to front -> raise(),show(),activateWindow() don’t work

try this:

viewer.setWindowState( (windowState() & ~Qt::WindowMinimized) | Qt::WindowActive);
viewer.raise();  // for MacOS
viewer.activateWindow(); // for Windows

it work in my project ( in my project viewer is QMainWindow): https://github.com/iptton/Rythem .

This problem is specific to Windows. If the active window belongs to some process, then Windows does not allow other processes to change the active Window.

(Do not try the following: https://wiki.qt.io/Qt_project_org_faq#QWidget_::activateWindow.28.29_-_behavior_under_windows)

for ( QWindow* appWindow : qApplication.allWindows() )
{
  appWindow->show(); //bring window to top on OSX
  appWindow->raise(); //bring window from minimized state on OSX

  appWindow->requestActivate(); //bring window to front/unminimize on windows
}

Note that this also brings up the window from other virtual desktops on both OSX and Windows. I did not test this on linux, it may work though.