Windows here in Europe don't usually slide. And in Europe we like looking out of them :)

But heat pumps are also available in single units now. They're getting quite popular because they can be self installed (no refrigerant engineer necessary).

I believe there are models of window units with heaters available though they may just be doing resistive heating. I've seen plenty of mini-splits running as heaters obviously using heat pumps and they seem to work quite well.