Comment 8 for bug 1248759

aha! If I use this code:

import QtQuick 2.0
import Ubuntu.Components 0.1

MainView {
    width: units.gu(50)
    height: units.gu(80)
    id: mainView

    Tabs {
        Tab {
            title: "test"

            page: Page {
                id: page
                title: "test page"
                Label {
                    anchors.centerIn: parent
                    text: "testing the toolbar"
                }
                tools: ToolbarItems {
                    id: toolbarItems
                    ToolbarButton {
                        text: "action1"
                    }
                    locked: true
                    opened: true
                }
            }
        }

        Tab {
            title: "empty"
            page: Page { }
        }
    }
}

and then switch tabs, and switch back, I can reproduce.