If there's a ledge beyond your window which your cat can land on and explore, it's best to not open the window or have a mesh covering it. Cats are curious creatures and they will explore every nook and cranny at home. I had my cat climbing out of the window once to the outside where the AC compressor is at. I thank god that there's ample space for me t o climb out else i would have to call the bomba. from that day onwards, we are very careful on how big we open the window. If you need to, maybe place bottles along the window sill if possible. That leaves them no space to climb or stand. They will not attempt to climb out then.