This is driving every. single. one. of. us. CRAZY. Whoever knows the code - I am shaming you! GO FIX IT NOW!
There isn't an obvious way to fix this without dramatically changing the way pages are rendered. The reason the link expires is probably that the garbage collector picks them up after a timeout interval. I remember this was something on the order of 10-30 min.
Honestly though, HN is a free service built with an experimental programming language buy a guy who worked full-time as a seed-stage investor. That's a miracle in itself. You're not going to "shame" anyone into fixing it.
We can see that it's an hidden field. So maybe an extension can re-fetch the page and updates the hidden field every 5-10 minutes, that would fix it for the HN users complaining about it.
This is just a cop-out. HN gets massive amounts of traffic and obvious problems like this one should be fixed.
PG spent _lots_ of time creating tools to better moderate content and block spammers, which are features that users don't see but actually make a huge difference.
When you're building a house, painting the walls is the last thing you do.
Is there any obvious reason why this isn't done?
Actually, it's easy to fix, because it's easy to turn dynamic urls ("fnid" urls) into regular URLs (something like "/submitreply?id=xxx").
They just haven't gotten around to it yet, which is fine.
(It's one thing if I end up with something that doesn't work in a test case I failed to consider beforehand, or doesn't fit in well with whatever plans exist on the part of HN's current maintainers; that's something I can understand, and I'm not about to get my nose out of joint over it. What would raise my ire would be to put in the effort and see it go zinging off into the ether, never to be heard from again.)
"It's on our list. There are a lot of things on the list; we don't know yet when we'll get to it."
So I imagine that if you submitted a pull request, they would at least look at it when trying to fix it.
I use the HN edit box. On mobile, I take care to copy my comment to clipboard before submitting it, so that if I hit the unknown/expired issue then I can hit the back button, refresh, paste, and resubmit. On desktop it's a nonissue because when it happens you can hit the back button and (in firefox/chrome) the edit box will have dutifully saved your whole comment, so nothing will have been lost.
I'd consider it more of a minor inconvenience.
Honestly it's nice to see an esoteric correct approach. If you're using a web "app" you shouldn't expect anything but a toy anyway. The only place it ever actually bothers me is paginated topics.
Probably the most straightforward way to 'fix' this would be to serialize delimited continuations to disk, but who knows if it would be easy to import that functionality into Arc.
On the other hand, C-a C-c C-r <submit>.. the mousing is by far the hardest part.