I could just as easily say that theft occurs via the threat of force, explicit or implicit: I don't resist your theft because, usually, I know or strongly suspect you have a firearm, or a knife, or a greater willingness to use your muscle mass to rearrange my organs.
So, if I resist your theft, am I asserting property rights or am I exercising my right to self-defense?
I'd personally be inclined to say that what you're doing when you resist my threat is contingent upon what kind of social arrangement we find ourself in.
In a Western-style parliamentary republic, your action will probably be interpreted as asserting property rights. If my intentions were obviously physically harmful, then we can throw in the self-defense, too.
But let's change the chair to something more consequential—a small crate of canned food. Let's say that you and I both know that you are not lacking for resources or food, and that I am homeless. And let's say that I steal this crate.
According to the interpretation of the NAP which takes property rights as a fundamental and does not admit of property already being an act of aggression in the first place, I'm the criminal, and you'll be in the right (maybe an asshole, but in the right) to resist my theft.
But assuming all the above, is it any more of an aggression to take your food (assuming I'm not off to stab you or whatnot) than for you to benefit from a societal interpretation of rights that threatens me with retaliation if I attempt to feed myself?
Perhaps it is easier to explore the limits of individual ownership when they are taken to the extreme, rather than to a minimum asshole status example.
Consider that an entrepreneur could obtain ownership of an entire nation purely through voluntary contracts that mutually benefited the parties involved. This differs from a despot only in justification of ownership being based in rights rhetoric rather than the divine. If the new king decides to shutdown all of the factories and farms so he can have more hunting land, what are the counterparties supposed to do? Lay down their plows and quietly consent to death?
At the point where individual ownership creates a power structure over other citizens' ability to use their labor to reproduce society's needs, that ownership has obligations of higher moral standing than the individual owner's whims. I tend to think preventing people from working to help themselves is even more abhorrent than merely hording resources, so better highlights the limits of ownership. Which power structures we consent to in our economic ecosystem and which individuals we offer to run them should always be up for renegotiation.
So, if I resist your theft, am I asserting property rights or am I exercising my right to self-defense?