When changing a slider value inside a nested Flickable, the bottom flickable can steal the mouse events.

[Nick Dedekind]

The slider position change event is not preventing the event from being stolen by a parent if the Flickable is inside another Flickable (somewhere in the tree). It's only looking at the first parent Flickable.
QtQuick.Controls.Slider works around this by using the preventStealing property.

Test Script:

import QtQuick 2.0
import Ubuntu.Components 1.1

Item {
    height: units.gu(50)
    width: units.gu(50)

    Flickable {
        anchors.fill: parent
        contentWidth: flickInner.width

        Flickable {
            id: flickInner
            width: units.gu(100)
            anchors.top: parent.top
            anchors.bottom: parent.bottom
            contentHeight: inner.height

            Item {
                id: inner
                height: units.gu(100)
                Slider {}
            }
        }
    }
}