Listed tabs as closeable but not close icon is shown

[HarryEllisCCQ]

I've defined the following tabs but when they're displaying there's no X shown to close them despite closeable being set to true

function newMainTab() {
    return {
        id: `MainTab${++count}`,
        title: `LSC Full Window ${count}`,   // title that's shown in the tab of the panel header
        closeable: true,
        content: <LineScansPage />,
        group: "closeAll",
    } as TabData;
}

function newViewerTab() {
    return {
        id: `BasicTab${++count}`,
        title: `LineScans Viewer ${count}`,
        closeable: true,
        content: <LineScansViewerOnly />,
        group: "closeAll",
    } as TabData;
}