That isn't seamless - windows has many different was to handle HiDPI, and for a few of them, the window will violently change size as it moves across and will look completely wrong on the "unmatched" monitor, to the point of being completely useless on that monitor (way too big to see anything or way too small to read anything).

What Wayland is doing is making it so the window looks the same size on all screens, with the source matching the primary or highest dpi monitor, and the composite scaling the content to match other monitors. This makes it equally useful on all monitors, at the cost of non-primary monitors having lower clarity.

macOS handles it well by cheating: a window can only be shown on one monitor at any given time. Only while moving a window will it temporarily be allowed to be seen on two monitors at once.