This morning, while we were having breakfast, my partner was busy with their phone. At that moment, if I’m not mistaken, they liked a photo of their ex right in front of me. I can’t understand how they can do that while having breakfast with me. Should I ask about it or just ignore it?
I think if you don’t ask this directly, it will weigh on your mind. Just ignoring it while continuing breakfast won’t make everything seem okay. You should find out what the guy’s intentions are, just wait and see 
this isn’t normal. I would get angry.
You might have seen it wrong, are you sure? If you’re not even sure, it would just be unnecessary stress for the guy.
@cokdaseyapma I noticed, but I can’t say for sure that I saw. After doing it, I didn’t even look at your face; I just froze for a moment 
@aklimkaldi if you’re not sure, and the guy denies it, the issue will seem closed but you’ll keep holding it inside. I think this won’t be resolved without easing your mind.
They might have pressed it out of habit, such things happen. But if they do this right in front of you, they’re either not aware or they’re comfortable. Both are questionable situations 
To fully evaluate the meaning of this behavior in the context of a relationship, you first need to talk with the other person about their thought model and expectations. Don’t make assumptions without asking them; mind-reading can damage relationships.
Everyone has a threshold. If you feel uncomfortable, don’t hesitate to ask questions. After all, this is your relationship standard.