While discussing https://github.com/amethyst/amethyst/pull/2118 it came up that the UI coordinate system is different from the screen coordinate system because someone was annoyed that the UI coordinates were not in sync with world coordinates in a 2d game.
Currently if you want to position some UI element on top of some world thing in 3d, you have to invert the y-axis of the output of world_to_screen. I find that very ugly.
If the UI coordinates were reverted, 2d would work just fine using world_to_screen.
But there is a solution that pleases all parties! The default orthographic camera currently mirrors y-axis. If we remove that mirroring, x and y point in the same direction in screen coordinates and the default 2d camera.