The property of something to return to its original state when not being acted upon

There are several words that apply here. The soup with the heavy stuff on the bottom and liquid at the top is in an equilibrium or a steady state. Sometimes, when you perturb an equilibrium, then remove the outside influence that was changing things, the system returns to its old steady state. So if you push a swing or a pendulum, just once, it will wave back and forth for a while and then eventually settle to hanging straight down as before. You stir the soup, it gets all mixed up, then you stop and it goes back to how it was.

But not all properties of a system depend only on their current state. In high school chemistry we learned that the viscosity (thickness or runniness) of, say, water, depends only on its current temperature - you can freeze and melt, freeze and melt, but 50 degree water will always have the same viscosity. Not true of an egg: if you boil it for ten minutes and return it to room temperature its viscosity is permanently changed.

And some systems return to a different equilibrium after a perturbance. Consider a piece of paper or wood. If you apply a little heat to it (say, a match) then it goes from being stable and just sitting around to burning. And it will happily continue to burn although the match is gone. In many cases when it has stopped burning the equilibrium it reaches is being a pile of ash and cinders. Or a burning piece of wood, if you blow on it to blow out the flame, won't go back to burning when you stop blowing on it.

In physics, you use the word elastic to mean that the shape changes with stress, then goes back to the old shape. (This is what an elastic band does.) The word plastic means that it changes its shape and stays changed, like plasticine or playdough. But if you want to use these metaphorically, tread carefully, because most people take elastic to mean "can be stretched indefinitely" and forget the part about going back to the old shape when the stress is removed. To use some more words from physics, resilient is popular for recovering back to the old state, and brittle for not doing so, like your smashed computer. And when you build a computer, you are removing a lot of entropy, getting it all ordered and carefully arranged. But in the absence of outside forces, entropy only increases. That's why the smashed computer won't go back to how it was.

Once you bring in the cows/grass example you are into predator-prey theory and not far from chaos theory, full of crazy words like saddles and strudels. There are stable equilibria, unstable equilibria, and all manner of words that mean a lot to practitioners but less in general conversation. I think your best bet is to describe one state of a system as a stable equilibrium while understanding that when you're being precise, an equilibrium can be stable to some perturbations (like stirring) but not to others (like being poured down the sink or being eaten.)

The examples you've used aren't really related, but two words you could consider are equilibrium which is a state of balanced forces - such as with your cows example or steady-state which is the state of being stable, which might apply to your soup example.

The property of the soup that causes it to maintain a steady state is inertia.

It's not a property of the soup but a property of the action, which is called a reversible change. In this case it is the fluid friction in the soup that returns it to its normal state, reversing your stirring action.