In the full site on mobile, long usernames overlap with the Profiles or the Network Profile button.
Examples:
This happens when the user only has "Member for { } day(s)/month(s)/year(s)" and "Last seen this week" under their username1. This does not happen when a user has "Last seen more than a week/month/year ago", "Member for {1-12} year(s), { } day(s)/month(s)" or any other information below their username.
1 In this case, the position of the username is shifted next to the profile picture, so a long username would then overlap with the Profiles/Network profile button..
Examples of this issue not happening:
I'm using Chrome for Android.