I know you work in airworthiness from your post history. It also indicates you work with flight systems, and not structures. Amirite? What is your professional qualification?
You can see what I do in my profile.
>What happens if you take a square with sides of length x, and smooth the corners down until the radius of each corner is x/2?
Same thing as happens with any regular polygon with sides of length x.
You said "It's not the shape of the window but the radius of the corner". This, like every other statement in your initial comment, is a misleading half truth at best. The true picture is complex, but the shape definitely matters, because you want to avoid abrupt local changes of stiffness in the direction of most stress (this can happen without any openings even, for example by adding a local bracket).
