Need to resize the window to make QwtPlotSpectrogram shows correctly

[Uwe]

What does "Qt::WA_WState_Polished" mean ?

It is an attribute, that is set once the widget has received a QEvent::Polish event - see https://doc.qt.io/qt-5/qwidget.html#ensurePolished.

A widget always gets an initial resize event that leads in case of the plot widget to calling updateLayout and there is no reason to trigger extra updates before that. So checking for Qt::WA_WState_Polished is a minor optimization for this initial phase.

what do you think about the fact that I need to set an expanding vertical and horizontal size policy for the QwtPlot which is a child of another QWidget so that the QwtPlot takes all the available space

A widget doesn't take any space it is always the parent that gives its children their geometries. So obviously your parent widget does this according to the size policies of its children. The default size policy of the plot widget is QSizePolicy::MinimumExpanding in both directions - why your parent does what it does then is hard to say without knowing your code.

Uwe