Posting to Zot groups/forums and posting to ActivityPub groups/forums use the same UI, but don't work the same
For posting to groups or forums on projects connected through ActivityPub, e.g. Lemmy, /kbin or Friendica, the same UI features are offered as for posting to Hubzilla groups. This does not mean they work for projects connected through ActivityPub the same as they work for Hubzilla and (streams), if at all.
For example, Friendica groups, Lemmy communities and /kbin magazines appear in the left sidebar of the stream page as forums. They can be clicked and read just like Hubzilla forums. They even offer a full-fledged post editor just like Hubzilla forums.
However, this post editor does not work the same on Friendica groups, Lemmy communities and /kbin magazines as it does on Hubzilla forums. If you write a post for a Friendica group/a Lemmy community/a /kbin magazine just like you write one for a Hubzilla forum, it will never arrive there, at least it will never be distributed. That's because unlike Hubzilla forums, they require to be explicitly mentioned in the message body.
Speaking of mentions, Lemmy communities and /kbin magazines offer auto-complete for private @! mentions, but @! mentions don't work on them; again, the post either never arrives or isn't distributed. Only public @ mentions work which, however, don't work properly on Hubzilla forums.
Friendica groups don't offer auto-complete for private @! mentions in the first place, only for public @ mentions which, again, are required to post to Friendica groups.
Finally, posting to a Lemmy community, a /kbin magazine or a Friendica group by selecting it in the permission settings in the bottom right corner of the post editor, which works perfectly well for Hubzilla and (streams) forums, does not work either. For one, again, the required mention is missing. Besides, even if a correct, working mention was there, the permission settings would still keep the post from either reaching its recipient or being delivered by the recipient.
My suggestion here is to unify the group/forum posting process which would improve the Hubzilla UX a lot.
The post editor on top of forum streams should be made to work with groups/forums connected through ActivityPub. For example, when starting a new post, a public @ mention could automatically appear in the editor if the group/forum that's to be posted to is connected through ActivityPub and requires a mention. Or if the post is sent, and the mention is missing, it could be added to the beginning of the message post, either in plain sight or behind the scenes. Either way, for groups/forums connected through ActivityPub, the post editor would stop just looking useful while being half-useless.
Also, something should be done about the issue of requiring two different kinds of mentions for Zot groups (private @! mandatory) and ActivityPub groups (public @ mandatory, private @! not even accepted), only one of each works in either case. Maybe posts that mention ActivityPub groups privately with @! should automatically be changed to a public @ mention.
At the very least, if nothing in this direction is implemented, auto-complete for private @! mentions of Lemmy groups and /kbin magazines should be suppressed like it already is for Friendica groups.
Finally, groups/forums connected through ActivityPub should be removed from the list of recipients in the per-post permission settings. Their presence is misleading if these permission settings don't work on them like they do on forums connected through Zot. And I can't see a way of fixing this without breaking permissions.