When commenting, having an autocomplete when you want to @reply to a user would be wonderful (similar to the one available on the desktop site). Normally when I'm on mobile site, I can copy/paste the username, remove the spaces, and I move on with life. But without copy/paste available within the app, the complex or unicode names are nearly impossible to use @reply.
Yes, it is really hard to type the username in comment box in Stack Exchange app. However showing username in a auto-complete dropdown menu is good for web-view whereas in app it may not look good(because no-where in app they used auto-complete dropdown).
Here is simple free-hand sketch of its look
Instead can we have add Reply option within Comment Options(to show difference between app and web) like this
Once they select Reply, then as usual we can move to the comment box showing