XenForo is definitely nicer than a lot of the other forum software out there I’ve used. It’s missing a couple of amenities that the old vBulletin had, but I think (but can’t remember for sure) that some of those were probably hacks we installed, and I know a lot of them (e.g., all the custom font formatting that users could customise posts with) were template edits that I instated myself, which I gather no one bothered to reinstate. (And some of the hacks are replicated by features that appear to be native to XenForo, like the post like system.*)
I think modern versions of phpBB may allow users to save drafts of posts, but they have to do that manually, and it doesn’t save some of the formatting; moreover, I don’t know if it allows users to save drafts of post edits. vBulletin definitely doesn’t allow saving of posts, or at least it didn’t in version 3.8.x, which was the last version I found to have a usable admin control panel, and that was ages ago.
It’s possible someone’s made an “auto-save post edits” hack for XenForo; I have certainly not gone through and looked to see what the XenForo modding community looks like. It probably wouldn’t be super difficult to simply steal a lot of the code from the base board software and adapt it for auto-saving post edits as well. But as a relatively experienced coder/admin, I can say that every time you mod a board, you run the risk of breaking something somewhere else. It requires a lot of testing and maintenance to mod board software, which is one reason admins are often suspicious of modding their boards too much.
I definitely have a lot fewer complaints with how this thing works than I’d have with modern versions of vBulletin (or older, less bloated versions of vBulletin, for that matter). It runs pretty smoothly and it’s got most of the features I’d want from a message board software – the ability to load new posts automatically is nice, and it’s got a really good notification system.
I wouldn’t object to a feature that automatically saves edits, but I suspect it would be most practical for the XenForo developers to make it themselves for a future version of the software; they undoubtedly have much better resources to test their own software than any third-party modder would.
One other thing I would add is that if you spend that long editing a post, it might be worth just making a new one. Double-posting is frowned upon in some circumstances, but it’s not a hard-and-fast rule. If you’re the last person to post in a thread, people probably won’t object to double posting as long as the new post is substantive and long enough has elapsed since the previous one. (In other words, if you’re just double-posting because you’re too lazy to figure out/use the multi-quote function, people will yell at you; if you’re double-posting because you’ve got a thousand words to add to something you already wrote, people won’t care.)
Anyway, for now, just use Lazarus or some similar software to save your edits, or save them in a text document as Ghost suggested. It’s kind of annoying that it takes third-party software to get that functionality, but I don’t know of any other message board software that would have that functionality either, and if any does, it’s probably got its own drawbacks. No software is perfect, since it’s developed by humans, who are intrinsically not perfect.
*…though I’m still moderately annoyed that none of the old thanks were imported as likes, since it means I need to go to the old board if I want to know who thanked the old posts. Not that I go re-reading old threads that often, but still.