If a QDialogButtonBox with no buttons is added to a layout, the sizeHint() of the layout containing the button box doesn't change after adding a button to the button box.

After show() is called, the size (height) of the layout is corrected.

I assume the problem is related to the dirty flag in QLayoutPrivate. This prevents a recalculation in QBoxLayout::sizeHint().

Probably adding the first button to a QDialogButtonBox should set the dirty flag of the layout that contains the button box.