dix and xwayland disagree about the '-wm' command line option
For most X servers '-wm' means default to setting window backing store to WhenMapped. For Xwayland it takes an argument naming which open file descriptor corresponds to the window manager client.
Honestly I kind of hate both things? There's no way to ask for Always instead of WhenMapped, and setting either one as the default for every window is, implicitly, automatic redirection at every child window, which seems super wasteful. I'd rather just stop accepting the option.
For xwayland it seems like that's trying to avoid the kind of race that !203 (merged) is trying to address, but it will fail to avoid that race in general (the wm fd will be added first, but there's nothing preventing Xwayland from accepting connections from other clients). weston will pass this option, but mutter won't; I don't know about other wayland servers. Again I think "stop accepting the option" is the way to go and we should fix !203 (merged) properly.