modificationChanged() signal will not emit after QTextEdit::setAlignment() called

[FinderCheng]

Hi, there! I wonder if you have found this problem. After I called
QTextEdit::setAlignment(Qt::AlignJustify), the textEdit->document()'s
modificationChanged(bool) will not be emitted. I've no idea if this is
my fault or something else. Here is my code:

MainWindow::MainWindow(QWidget *parent) :
   QMainWindow(parent),
   ui(new Ui::MainWindow)
{
   ui->setupUi(this);
   setWindowTitle(tr("Text[*]"));
   ui->textEdit->setAlignment(Qt::AlignJustify);
   connect(ui->textEdit->document(), SIGNAL(modificationChanged(bool)), this, SLOT(setWindowModified(bool)));
}

MainWindow::MainWindow(QWidget *parent) :
   QMainWindow(parent),
   ui(new Ui::MainWindow)
{
   ui->setupUi(this);
   setWindowTitle(tr("Text[*]"));
   ui->textEdit->setAlignment(Qt::AlignJustify);
   connect(ui->textEdit->document(), SIGNAL(modificationChanged(bool)), this, SLOT(setWindowModified(bool)));
}

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If I remove ui->textEdit->setAlignment(Qt::AlignJustify);, everything
is OK. But if this line is not removed, the window title will not be
changed. I think this is because the signal is not emitted. How do I solve this?