I have been trying for a while to thoroughly ignore any passive aggressive tone and take every word she says at face value. I absolutely pretend she means precisely what she is saying, never clarifying or arguing the tone or taking offense. When she huffed out of the house, she said (in a wounded tone) "Well, I am going to go now." I replied in a cheerful tone "OK, drive safely! Merry Christmas!"
Also, she and my first husband hated one another, since each believes him/herself to be the center of the universe. My upgraded husband is very mild mannered and agreeable, and my mother has been happy to feel vindicated by getting along with him. That gives him some power, and gives his cool "OK" a lot of weight.
But really, I have tried everything and never had reasonableness as the outcome with her. I hope it can be repeated, my life could use more non-stressful interactions.