This is one of my main points: good UX takes some thought. It's harder than doing the bare minimum. In this case, preserving someone's post in a <textarea> is the same concern whether it's in a modal or stuck at the bottom of the page. Neither using a modal nor not using a modal solve this for you.
It usually makes sense to introduce a localstorage draft system regardless of where the <textarea> appears on a forum.
This is one of my main points: good UX takes some thought. It's harder than doing the bare minimum. In this case, preserving someone's post in a <textarea> is the same concern whether it's in a modal or stuck at the bottom of the page. Neither using a modal nor not using a modal solve this for you.
It usually makes sense to introduce a localstorage draft system regardless of where the <textarea> appears on a forum.