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When having a frameless window with the Qt::FramelessWindowHint and the Qt::WindowSystemMenuHint flags set then it is possible to activate the window by clicking the application's taskbar button, but it is not possible to minimize it by clicking the taskbar button a second time.

Any other application will get activated when clicking it in the taskbar and then get minimized when clicking a second time.

Ans:
To be able to minimize a frameless window you need to add Qt::WindowMinimizeButtonHint

source link:http://bugreports.qt.nokia.com/browse/QTBUG-4262