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;
});