Can sliding windows be properly insulated? In general cold is a bigger deal in Europe than warmth, and will continue to be so.

German style tilting windows close as tightly as the regular (or door-like as you say) do. UK has windows sliding up, but is also famous for being drafty as the windows are never tight. I suppose good sliding windows can exist though?

I have myself pondered the problem with regular windows and a movable AC. My apartment has old school 4-pane windows with 2 layers both having their own window handles, so 8 independent small windows for each opening. They do look great in an old building but I don't see any reasonable way to set up AC with these. Thankfully no need yet as the apartment has never reached 30C inside, but we'll see what the future brings.

> Can sliding windows be properly insulated?

Yes. Beyond that, if they didn't work they wouldn't be used. Continental climates get much colder than pretty much anywhere in Europe, outside of a select few areas.

> In general cold is a bigger deal in Europe than warmth, and will continue to be so.

Masonry is a bad match for cold. The structure acts as a high velocity heat conduit and the earth eats all the heat you produce. Europe's winters (in general) are extremely mild, arguably even more so than its summers.

The old sliding windows were shit, but the modern ones are pretty good.