test_window_fullscreen.cpp is a separate test program with its own main(). It includes SDL_main.h and is built with SDL_MAIN_HANDLED. The single-owner guard sdl3_main_single_include counts every file under runtime/tests, so it has failed since that test was added.
Name the file as a stand-alone program. Every other file under runtime/src, runtime/include and runtime/tests is still held to the one-owner rule.
SDL_main.h is a single-header implementation in SDL3. Include it only from the translation unit that owns main so MinGW emits one WinMain instead of one per SDL-using source.
Matthew Stanley