In one of my windows I've placed QVBoxLayout with three QGroupBox inside. Each of QGroupBox posses his own QVBoxLayout in which resides custom widgets. QGroupBox are set without MaximumSize.
Now - if I click on custom widgets - the should show extra QLabel below. But those lines are ... painted vertically squeezed.
It looks like they are trying to be painted in place previously used only by one line. Hmm - maybe they need QSizePolicy? So for every QGroupBox I've set verticalPolicy to Expanding (so they should be shrinking and expanding when need).
Almost no change. Now when I click - for a second space is prepared - but then everything is collapsing to state before.

If I understand correctly - I need to say to QGroupBox in which one of item should expand to increase his size. This QGroupBox should say to Layout in which he resides - I need more space. But don't know how? I never use setMaximumHeight - only twice setMinimumHeight (for max height of QLabel). In one QGroupBox there are 8 items, in second - 4 and in last one only ONE. So place is enough to expand.