After fighting for 2 years to hav e my Toshiba TV not automatically turn off on me despite sleep timer and power saving are both off. (see suggestion 1, 8 and 11), it keeps giving me a warning after a while that the TV is powering off if I am not pressing a key on the remote. This is especially annoying as I connected it to my PC as a 3rd monitor.
The solution is found in the developer mode of the TV as described here.
It is perhaps done to avoid burn-ins (which isn’t just a CRT thing as Linus Tech Tips describes here)
Still this sucks if you don’t know about it and usually nothing much happens on your 3rd screen if ever.
Android Developer Mode for the win!