The taskbar button requires the window handle to be set with a call to button->setWindow(mainWin->windowHandle()). This doesn't work if it's called from mainWin's constructor.

It's possible it doesn't work until mainWin itself has been shown.

See http://stackoverflow.com/questions/25916966/what-am-i-doing-wrong-with-qwintaskbarprogress

Neither the documentation for QWinTaskbarButton (neither the example code nor QWinTaskbarButton::setWindow()) nor the documentation for QWidget::windowHandle() mentions anything about this.

Figuring this out the hard way is very difficult and time-consuming.