Second to this, but actually primary to everything, if truly embracing "columns" a la Harmony.
Then we all should be thinking about breaking out features into the column area especially for composer.
Then alternatively, you can have a feature like tags vertically display in the left column in line with the composer edge, what I'm talking about here would be better considered as Composer-columns that overlay (if that technically makes it easier or is more efficient).
Bring/Give columns to composer. Space for breaking out features.
IMHO this is a more correct justification for column layouts, to provision for many cool secondary features even for other aspects of NodeBB (again widget in the main columns, is in line with this thinking too) that enhance posting beyond what is typically found in forum platform software.
Give composer wings! 💸