У троллей по поводу этого класса написано:
This class is obsolete. It is provided to keep old source working. We strongly advise against using it in new code.
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Qt 3.0 includes a new richtext engine which obsoletes QMultiLineEdit. It is still included for compatibility reasons. It is now a subclass of QTextEdit, and provides enough of the old QMultiLineEdit API to keep old applications working.
If you implement something new with QMultiLineEdit, we suggest using QTextEdit instead and call QTextEdit::setTextFormat(Qt::PlainText).
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Some odd usage will require some porting. In these cases, it may be better to use QTextEdit now.
Производитель рекомендует для этого использовать новый класс QTextEdit вместо устаревшего QMultiLineEdit. Чтобы работало все в цвете и красиво, надо установить формат текста:
QTextEdit::setTextFormat(Qt::RichText);