QTextEdit / QPlainTextEdit

connect(textEdit, &QTextEdit::cursorPositionChanged, this, [](){
    qDebug() << "カーソルが移動しました";
});

QLineEdit

connect(lineEdit, &QLineEdit::cursorPositionChanged,
        this, [](int oldPos, int newPos){
    qDebug() << oldPos << "->" << newPos;
});