I have two displays, a 1080@144 and a 4k@60. I prefer the 144 hz display for playing games on, and also launching fullscreen apps I’m developing so I can keep my editor/debugger/whatnot on the high-res display. To force that in F37 (or other distros), I set my 144 hz display to be the primary display in Gnome Settings. In F38 it always defaults to placing them on my (non-primary) 4k display. In many SDL based games, it’s impossible to run them fullscreen on my 144 hz display, as setting them to fullscreen always picks my (non-primary) 4k display instead.
The only workaround I’ve found is to set SDL_VIDEODRIVER=x11 as XWayland seems to pick the correct display, but it only works for SDL games. I’m aware that Fedora forces Wayland for SDL games. There used to be a few SDL/Wayland issues with games, but overall I wasn’t bothered by it. Since launching fullscreen is the default for most games, this now breaks nearly all games. 
Unsure where to file this bug though. It doesn’t seem to be an SDL bug as I tested the latest release from source on both F37 and F38. Could this be a Mutter regression or is it something that changed in Wayland?