It seems to be isolated only to links to other Discourse instances though.
Repro steps, with “Open external links in new tab” enabled in options:
- Reply to a post containing a link to another Discourse instance (not a oneboxed post)
- Click the “Reply to” indicator.
- Click on the link
- external links to non-Discourse sites seem to work fine
- links to other Discourse instances open in same tab
- EXCEPTION: http://what.thedailywtf.com behaves correctly
Reproduced here in a PM chain.