[KevinH]

I will try that approach.

But the code that does each are almost identical:

From BookBrowser:

void BookBrowser::ValidateStylesheetWithW3C()
{
    QList <Resource *> resources = ValidSelectedResources(Resource::CSSResourceType);
    foreach(Resource * resource, resources) {
        CSSResource *css_resource = qobject_cast<CSSResource *>(resource);
        Q_ASSERT(css_resource);
        css_resource->ValidateStylesheetWithW3C();
    }
}

And from MainWindow

bool MainWindow::ValidateStylesheetsWithW3C()
{
    SaveTabData();
    QList<Resource *> css_resources = m_BookBrowser->AllCSSResources();

    if (css_resources.isEmpty()) {
        ShowMessageOnStatusBar(tr("This EPUB does not contain any CSS stylesheets to validate."));
        return true;
    }

    foreach(Resource * resource, css_resources) {
        CSSResource *css_resource = qobject_cast<CSSResource *>(resource);
        css_resource->ValidateStylesheetWithW3C();
    }
    return true;
}

So I realy can not see why one would work and the other one not.

I tried this on MacOS and it worked properly all the time, both ways.

When I get a chance I will try it on Linux and report back.