I see: you are uneasy about interrupting someone, and you feel you might as well do it gently. Break the ice, prepare the turf, something like that. You are asking that guy a favor, a question, and you don’t want to overwhelm him right off the bat. Except you might be doing something worse: interrupting that person to notify him that you’re still typing whatever it is you want to send his way.
Slack has announced a new feature called Threads, aimed at taking the clutter out of public channels. At first, you might think that Direct Messages already covered that, but it’s not so.