this is clearly a big issue and problem !! I, d like to thank @dana-edwards for posing as ever the good questions here !! One of our most valuable assets for thought provoking posts in my opinion on the Steemit platform of exchange.
Idea,s are yours until you expose them to others no? So if you post an idea on the internet or even upload a painting or music piece to the cloud for " safe storage " you have already exposed your idea and sadly i feel you risk to lose it !! The best and only way of keeping true and brilliant idea,s or creations is to keep them like your trezor wallet, offline in a draw or in your mind. But the day you chose to try and develop your idea,s and you wish to market it anywhere, the moment of risk clearly arises as you will be forced really to expose them to others " " Hell is the other,s !! " Jean Paul Sartre.
I think that plagiarism is an old concept which existed well before the internet technologies, but the internet greatly advanced the process and accelerated it to the point where your idea in a matter of hours could be taken on by somebody else thousands of miles away and with little hope of you ever proving to anyone that their creation is in fact yours .