I noticed that too!! I'm not entirely sure why (so maybe I shouldn't be posting..??!!), but I do seem to recall that they had something going on a while back but decided that they were too much like friends to make a go of it!
Some time after Drew (Libby's hubby) died, Stuart - friend of Drew - moved to Erinsborough, and had a thing for Libby but she didn't have a thing back, hence the kiss. OK?