If you plonk a QSpinBox (or any QAbstractSpinBox sub-class) onto a widget with the Qt::Popup and Qt::FramelessWindowHint flags set in combintation with the Qt::WA_TranslucentBackground attribute, the spin controls flicker a horrible black colour when the mouse enters/leaves them on Windows.

I've attached a small example app that shows this going wrong. Start the app, click the big button and then move the mouse cursor over the spin control in the pop-up widget that appears.