deleting frames in QTextEdit

Hi
I am experimenting with QTextEdit and frames.
I have a small app in which I insert frames in response to a menu action.
The frame gets inserted correctly but when I delete text using the backspace key I can only delete to the beginning of the frame.

Does anyone know the reason for this?

the code that inserts the frame –

Code:

---

void MainWindow::on_actionFrame_triggered()
    {
        QTextCharFormat charFormat;
        charFormat.setFontPointSize(20);
        ui->textEdit->textCursor().mergeCharFormat(charFormat);
        ui->textEdit->textCursor().insertText(QString::number(m_frameNumber++) + "Frame");
        QTextFrameFormat frameFormat;
        ui->textEdit->textCursor().insertFrame(frameFormat);
    }

---

Thanks

Graham