I've seen potential flare-ups before like on the Broadwater (Gold Coast) when the current is flowing fairly fast and one boat is anchored and another is drifting not too far from shore and when they cross paths within 3 metres or so of each other there are moans and groans and winges from both boat crews about who should give way or be getting their lines out of the water or to "pi## off" etc.
just a point when i was reading your post PNG1M
I'M FAIRLY SURE that a vessel that is drifting is deemed to be underway.
in that case it must giveway or show clearance to the vessel at anchor.
probably would be different if they were anchored in a navigation channel and we have a vessel restricted by draft or manouverability
i know of a case of a 50 ton trawler colliding with a tinny drifting across its path
worth thinking about because that incident ended with overturned manslaughter charges
regards
ken