Nice tool, thanks for creating. My feedback so far is: 1) Consider pre-filling the box with [[User:usernameGoesHere]] or similar, to trigger a ping. This is useful on mobile, where it is easier to delete a prefill than it is to type all that out. The user script reply-link does this on enwiki. 2) Keep an eye out for a bug where you put the cursor in one place, but when you type, the text goes in a different place. I had this happen on Android the other day, one time, but haven't seen it since. 3) If this tool is still not available in beta on certain wikis, consider documenting the following a little bit better, so people can opt in: if ( $( '#ca-addsection' ).length ) mw.loader.using( 'ext.discussionTools.init' ); . Again, great job.
Topic on Talk:Talk pages project/Replying/Flow
This is absolutely the top feature needed! Especially for newer users who'd benefit from talk page discussions but are not as wiki syntax savvy. I am very happy with this!
I just filed phab:T290198 about the interaction between pinging and Talk pages project/Notifications. That detail will hopefully get fixed, but more generally, I wonder if we will someday ping people a lot less often. If everyone's already getting a notice about every discussion, then why would I bother pinging them?
If you haven't played with the notifications system yet, then go to Wikipedia:Village_pump (technical)#dtenable testing and click the [subscribe] button.