https://github.com/qtproject/qtwinextras/blob/dev/src/winextras/qwinthumbnailtoolbar.cpp#L512

method void QWinThumbnailToolBarPrivate::_q_updateToolbar() :

QtWin::toHICON uses CreateIconIndirect winapi call, deletion must be performed by DestroyIcon, but freeButtonResources(buttons) at the end of the method uses DeleteObject. Task Manager also shows that gdi objects are increasing. After commenting out QWinThumbnailToolBar gdi leak dissapeared.