Signals undoAvailable(bool) and redoAvailable(bool) shouldn't get emitted if the QPlainTextEdit has readOnly to true.

See Example of this here:

#include <QWidget>
#include <QPlainTextEdit>
#include <QPushButton>
#include <QVBoxLayout>

class Widget : public QWidget
{
Q_OBJECT
public:
Widget();
public slots:
void redoAvailableDebug(bool aval);
void undoAvailableDebug(bool aval);
void changeText();
private:
QPlainTextEdit *textEdit;
QPushButton *button;
QVBoxLayout *layout;
};
#include <QDebug>
#include <time.h>
#include <QTime>

Widget::Widget()
{
textEdit = new QPlainTextEdit();
button = new QPushButton("Change Text");
layout = new QVBoxLayout();

layout->addWidget(textEdit);
layout->addWidget(button);
this->setLayout(layout);

textEdit->setReadOnly(true);

connect(textEdit, SIGNAL(redoAvailable(bool)), this, SLOT(redoAvailableDebug(bool)));
connect(textEdit, SIGNAL(undoAvailable(bool)), this, SLOT(undoAvailableDebug(bool)));
connect(button, SIGNAL(clicked()), this, SLOT(changeText()));

}

void Widget::redoAvailableDebug(bool aval)
{
qDebug() << "redoSignal Called and " << aval;
}
void Widget::undoAvailableDebug(bool aval)
{
qDebug() << "undoSignal Called and " << aval;
}

void Widget::changeText()
{
QTime curr = QTime::currentTime();

textEdit->insertPlainText(curr.toString());
textEdit->undo();
}

#include <QApplication>

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Widget widget;
widget.show();

return a.exec();
}

#include "main.moc"