@mpergand Thanks - this is what I've done but it's not ideal as there two boxes have different "natural" widths as it were, and forcing a proportional size on them is a bit of a kludge. Better than not - but you'd like to think it it shouldn't be needed.

Can you run a little test ?

What do you get if you call qDebug() << yourToolBox->sizeHint(); before and after it's shown ?

Do this for the widgets inside the ToolBox:

widget->setBackgroundRole(QPalette::Base);