Want (and type):
ui->plainTextEdit->textCursor().insertText(QString("ПК номер %1 на %2 заражён! "
                                                   "Шаг: %3\n").arg(id).arg(pcs[id]->os).arg(step - 1));
What became after saving changes (Qt initially does as I want but thi changes after save/compile):
ui->plainTextEdit->textCursor().insertText(QString("ПК номер %1 на %2 заражён! "
                                                   "Шаг: %3\n").arg(id).arg(pcs[id]->os).arg(step - 1));