This is about good manners and taking accountability for one's actions (or not).

MSObelle, I floss, too. Twice a day. I'd like to keep my teeth until they plant me, given, among other things, that I wore braces for five years as a child.

That said, there is a time and place for everything. And flossing one's teeth seated in an aircraft cabin (and not in the lav, the appropriate venue for such activity - sorry, OP) is, IMHO, lacking in sensitivity and good manners. Whether you spray food particles all over or not matters not.

All the things you mentioned, e.g., farting, nose-picking, uncovered sneezing, yes they happen and they are no more unacceptable than flossing. The issue here is the brazenness of the flosser thinking it's okay for her to do what she did. But then again, when the FA showed up she immediately stopped. Which suggests to me that she knew it wasn't appropriate.