I actually implemented what you suggest in https://github.com/mozilla-b2g/gaia/pull/16377/files#diff-11641ddcb5258b7f7ab2483afb800726R153

I remember I did what you're suggesting at one point but that I then reverted my change, because it involves more changes in ThreadUI as `ThreadUI.onMessageSending` is currently meant to be called only if we're not transitioning. I thought it would be easier to do a more chirurgical patch to change only this in a next small patch.

Does it make sense?