[KevinH]

Okay, this needs to be handled in FlowTab.cpp SplitSection() before the OldTabRequest is fired as that is obviously too late.

void FlowTab::SplitSection()
{
    if (!IsDataWellFormed()) {
        return;
    }

    QWidget *mainWindow_w = Utility::GetMainWindow();
    MainWindow *mainWindow = qobject_cast<MainWindow *>(mainWindow_w);
    if (!mainWindow) {
        Utility::DisplayStdErrorDialog("Could not determine main window.");
        return;
    }
    HTMLResource * nav_resource = mainWindow->GetCurrentBook()->GetConstOPF()->GetNavResource();
    if (nav_resource && (nav_resource == m_HTMLResource)) {
        Utility::DisplayStdErrorDialog("The Nav file can not be split");
        return;
    }

    // Handle warning the user about undefined url fragments.                                                                   
    if (!mainWindow->ProceedWithUndefinedUrlFragments()) {
        return;
    }

    if (m_wCodeView) {
        emit OldTabRequest(m_wCodeView->SplitSection(), m_HTMLResource);
    }
}

So from this, I think we can simply remove the "Cannot be split" from that part of MainWindow.cpp as it is too late by that point to not hurt something.

We can then "*assume*" that a .xml file that actually gets successfully loaded into a FlowTab itself must be xhtml (otherwise it would be loaded in a XMLTab) and maybe even remove that test in MainWidow completely since it is already too late.

I will try doing that.