It means that she is thinking of a nasty thing to say before she leaves.

The metaphor here (parting thrust) is sword fighting. What this phrase basically means is that she is trying to decide upon a suitably insulting thing to say as she leaves (which the author compares to a well placed stab which ends a sword fight).

It sounds to me like she had a dramatic exit. She got ready to "compose" ....this would mean she created a certain posture and somehow went out the door in a creative manner.....I would not know the personality of this person, but it must have been with drama or silliness.

you say it's a reference to her sexual encounter, according to the story content, it could possible be, but what does "composing her parting thrust"mean, could you explain in detail, please, thanks a lot.

Wrong, wrong and wrong! It is a reference to her sexual encounter just before she left.