How to deal with petty anger?
Petty anger is resentment.
From DN33 we are told:

Nine grounds for resentment. Thinking: ‘They did wrong to me,’ you harbor resentment. Thinking: ‘They are doing wrong to me’ … ‘They will do wrong to me’ … ‘They did wrong by someone I love’ … ‘They are doing wrong by someone I love’ … ‘They will do wrong by someone I love’ … ‘They helped someone I dislike’ … ‘They are helping someone I dislike’ … Thinking: ‘They will help someone I dislike,’ you harbor resentment.
Nine methods to get rid of resentment. Thinking: ‘They did wrong to me, but what can I possibly do?’ you get rid of resentment. Thinking: ‘They are doing wrong to me …’ … ‘They will do wrong to me …’ … ‘They did wrong by someone I love …’ … ‘They are doing wrong by someone I love …’ … ‘They will do wrong by someone I love …’ … ‘They helped someone I dislike …’ … ‘They are helping someone I dislike …’ … Thinking: ‘They will help someone I dislike, but what can I possibly do?’ you get rid of resentment.

I can confirm through personal experience that the above does in fact work in everyday life. Memorize and repeat.