010101 actually, I already wrote a feature similar to that in https://github.com/flagrow/mason/pull/4
I just never released it because I haven't had time to test it fully. It's so much trickier than the current behavior and deals with mobile devices so that I'm pretty sure there are still issues to fix before releasing...
If you're interested to try it out (preferably on a local Flarum install) you can install the mobile-fields
branch of the extension to a workbench directory or might be able to install version dev-mobile-fields
via Composer (composer require flagrow/mason:dev-mobile-fields
I suppose).
Of course I could also add an option to just move the fields to a modal whether or not they would fit in the editor. This just couldn't be the default. The main problem is for people using it as mandatory fields. The user experience to ask to complete fields is not very good if you can't see the fields. I never tried if it was possible to open the modal if there are unfilled fields, like for the tags. It's possible it won't play well with the tags modal opening as well...