I was looking for similar bugs and saw this one but I didn't thought it's related as it's about crashing.
The selection in xf.com is definitely better than any other xenforo site that I was testing, though I noticed now that there are still cases which it selects all.
I've tested with 8.0.1. Now upgraded to 8.0.2 and it's the same.
Both of these are internal browser issues that we really have no control over. It relates to "anonymous" block elements being created to contain inline elements. The issues have been reported to Apple and I believe at least one of them was tagged as a duplicate, but their issue tracking is mostly internal so unfortunately we don't have a lot of information.