On viewtopic pages, each post has a title link which should link to the post itself, and this link is the obvious one to use a a permalink, or shareable link for the post. I find that these links are often pasted into other posts (or even on other sites as links to interesting posts on our forums.
These links currently have a format like:
or for a topic with multiple pages:
This works fine for the user who generated the link, but the &start= parameter can be calculated differently for different users, for example due to soft-deleted or un-approved posts (and maybe other reasons), so that the linked post is actually on a different page than the one selected by the &start= parameter. and the link thus takes some users to the wrong place.
This can easily be fixed by changing the link code in viewtopic_body.html to use an absolute link rather than the current relative link. The absolute post link is already available in the template.