>For example, X still has problem with subpixel rendering because there's no way for the client-side library to know the current display's orientation. That's why KDE and GNOME have separate manual settings for them, while Wayland carries this information in the native protocol.
What happens - either in X or Wayland - if one window spans displays which are not in the same orientation?
This monitor setup is not a hypothetical scenario for me BTW.
(And as an aside, I still think subpixel font rendering is a totally stupid idea, in part because of this very issue which means that the font renderer needs to have precise knowledge about the specific output device at any time; that sounds to me a lot like 'unwarranted chumminess with the implementation'. Leave aside what happens if there are any display transformations in effect, or if you want to create screenshots, or...)