Print: Preview.app opens before pdf is created

- (void)printPanelDidEnd:(NSPrintPanel *)printPanel
        returnCode:(int)returnCode contextInfo:(void *)contextInfo
{
...
 else if (dest == kPMDestinationPreview) {
            static QTemporaryDir printPreviews;
            auto documentName = printer->docName();
            if (documentName.isEmpty())
                documentName = QGuiApplication::applicationDisplayName();
            auto fileName = printPreviews.filePath(QString(QLatin1String("%1.pdf")).arg(documentName));
            printer->setOutputFileName(fileName);
            // Ideally we would have a callback when the PDF engine is done writing
            // to the file, and open Preview in response to that. Lacking that, we
            // use the quick and dirty assumption that the the print operation will
            // happen synchronously after the dialog is accepted, so we can defer
            // the opening of the file to the next runloop pass.
            dispatch_async(dispatch_get_main_queue(), ^{
                [NSWorkspace.sharedWorkspace openFile:fileName.toNSString()];
            });
        }
...
}