I've though about the code implementation - for anyone interested patch-settings shows how you can persist things in localStorage (this is how your theme and language are set locally).

I think the remaining piece would be to make a key-value pairs, where:

• key = a unique id fo the thread you're commenting on (probably just the message id)
• value = what you've written so far.

Then once you post, you wipe that draft.

If anyone would like to code this I'd be happy to support you xx