Test case attached, compile with:

g++ -o test test.cpp `pkg-config --cflags --libs Qt5Widgets` -fPIC

Sequence of events:

button->setVisible(false)
toolbar->addWidget(button)
button->setVisible(true)

The result is that the button is not shown.

Seems that calling addWidget with a hidden widget sets some internal state on the widget that causes subsequent calls to setVisible(true) to not have any effect. The WA_WState_Hidden bit is still set on the widget's attributes at the end of this test.

Tested in 5.2.0, 4.8.1 and 4.8.5, with GCC 4.4.6 on RHEL6.

Thanks