Rounded QComboBox without square box behind

Question

I'm styling a QComboBox in Qt. It is rounded how the figure shows. The problem is that is shows a strange square box behind the rounded border.

Can someone tell me what this box is and how to make it invisible?

By the way, I'd like to take the shadow away too.

Here is my current code:

QComboBox {
    border: 1px solid gray;
    border-radius: 10px;
    min-width: 6em;
}

QComboBox:on {
    border-bottom-left-radius: 0px;
    border-bottom-right-radius: 0px;
}

QComboBox QAbstractItemView {
    border-bottom-right-radius: 10px;
    border-bottom-left-radius: 10px;
    background: white;
    border: 1px solid gray;
    box-shadow: transparent;
}

QComboBox::drop-down {
    border-color: transparent;
}

Can somebody help here?? Thanks!