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.