Discourse seems to handle things better and is constantly talking to the server to check for new replies as you write yours, so I expect it won't have problems with sessions timing out while you write a reply.
But do keep in mind that if you spend more than a few hours writing a reply, any attachments you uploaded for the post may be deleted and no longer work. (The forum periodically checks for uploaded attachments which no post is using and deletes them.) So if you leave a reply open for several hours it can cause problems, but having one open for a few minutes or even a couple of hours should be fine.
(Although if you are writing something that long and detailed, saving it into a text editor first is probably a good idea. The server might be down, especially at the moment while we are still having to restart it to change certain things. Or the internet might go down, etc.)
On phpBB you can normally hit Back in the browser and recover what you had typed, but not always and it was indeed a pain when that happened.
Yes, exactly. All those broken imageshack links on old posts should be a thing of the past.
I had started to do this manually after the problems with imageshack, so I'm really glad the forum automates it.
As an aside, Discourse has a much better backup process than phpBB (which was a complete joke, didn't include files/attachments, and sometimes corrupted the SQL backup if it took too long to generate). So if anything catastrophic happens we should have more recent backups of the forum now than before. Backups are now automated, and can also be done by a single button push. With phpBB it took about an hour to do all the actions and checks necessary to do a full backup (posts + attachments) of the forum. phpBB was garbage.