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When posting a message and receiving a response, it can be a bit annoying to reply if the display name and username are not the same( for example : display n'aime : SAMI username :@238SAMIxD).
On a computer, it's simple - I just reply and type the '@displayname', and it automatically suggests their username.
However, when using a phone, it can be more difficult. I have to check their username, try to remember it, and it can be even harder if there are numbers involved.