Save Draft, or at least warn that message text will be lost.
I was typing a lovely message in conversations to collaborate on a class group assignment when I needed to reference information from another area of Canvas (Chapter number of readings). So when I closed the unsent message, whoosh, it was gone.
If a Save Draft feature is not easy or preferred, at least give a quick warning that when I hit Cancel in a message, the content will be lost--and a 'Don't show me again' option on the same warning box.
This suggestion is particularly geared towards mobile app users, who cannot simultaneously view more than one part of Canvas at a time, and for whom copy/pasting often requires very precise dexterity control. Moreover, these days, people are often accustomed to super-agile cloud-based auto-save; and I think this feature would follow the expectations of most message writers--even this Idea is being auto-saved as I speak!