Clicking on the "Paris" ComboBox in the gallery example under the Styles tab does not cause the items to drop down. The issue is only present with the example; other uses of ComboBox work:

import QtQuick 2.0
import QtQuick.Controls 1.0

Rectangle {
    ComboBox {
        anchors.centerIn: parent
        model: [ "Banana", "Apple", "Coconut" ]
    }
}

There is also a warning printed than can be reproduced with any ComboBox:

No suitable 'textRole' found for ComboBox.

There is a small grey rectangle drawn at the bottom left corner of the ComboBox that was not captured by the screenshot, which is presumably a small portion of the drop down itself.