How does the prius do on highway ?

I'm not sure I completely understand the question.

On the highway at a steady cruise at normal speeds on level ground, the car would typically charge the battery since it wants to run the engine at the power level that gets the most out of your petrol. That throttle opening is usually more than you need for that speed, so the excess energy goes into the battery until the battery gets so full that the car decides it better use that electricity so it has room if you slow down.

If you take your foot completely off the accelerator, the car will coast and put kinetic energy into the battery (charge the battery) to simulate the slowing you get with a regular gasser. The amount and direction of flow of the electricity will vary with the throttle opening, the battery's state of charge, the slope of the road, the wind, the engine temperature, and a boatload of other factors.

If something is wrong, the car will announce it with a warning light or message. This is a car that's both extremely care-free and also extremely fascinating.

 

From your logic, it shouldn't charge the battery when you take your foot off the accelerator because the battery is full.