If the word is spelled incorrectly and the auto spell check is active, any misspelled words will be underlined in red. Simply right click the underlined word for a list of suggested alternatives, or the option to add to your dictionary.
I'm not sure what is going on with your installation as mine is quite happy to keep the underlinings over a multi line multi paragraph posting. Not helpful I know, but there's not much else I can offer in this case, sorry.
This thread is now locked and can not be replied to.