This crude caching mecanism is useful in the particular case of a screen with many misspelling dotted lines. In this case, it is necessary to build a QTextLayout in order to know where to put the start/end of the spell line. Since rows typically contains text snippets longer than a word, we may be in a situation where the same QTextLayout is constructed repeatedly.
This commit is useful in this particular use case, and should not be costly in other cases. A better fix would be to remember the QTextLayout associated to each row element. This is a bit more work, so this fix should be sufficient for now.
Additionally, do not paint misspelled marks when painting is disabled.