User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/83.0.4103.119 Safari/537.36

Steps to reproduce:

I am trying to convert an existing legacy Addon to the new Mailextension API and miss the feature to add tags.
It is possible to query existing tags with messenger.messages.listTags()
and add existing tags to a message with messenger.messages.update(...)
but it is not possible to add new tags to the list of available tags.

Actual results:

To add a new tag to the list of available tags the user has to add the tags manually in the options menu.

In the legacy API it was possible to add tags through the tagservice:
Components.classes["@mozilla.org/messenger/tagservice;1"].getService(Components.interfaces.nsIMsgTagService);

Expected results:

It should be possible to add tags with an Mailextension API call like it was possible with the legacy tagservice.

Will this API be implemented or not?

Some feedback would be nice.

I made an simple experiment API which implements the feature to add tags.

https://github.com/int-red/experiment-api-tagservice

Pushed by mkmelin@iki.fi:
https://hg.mozilla.org/comm-central/rev/861b6a17f926
Add API to create, modify and delete tags. r=mkmelin