[...]
Add to user notes content of X selection when emacs is started as
daemong with no frames and the user prefers to avoid distraction due to
creation of a new frame.
In general, you seem to assume that there's no way for emacs to create a new X11 frame object without it being immediately visible to the user. In my experience, that's only true in the limited subset of cases where the user has chosen a window manager that enforces that choice. Further, that wasn't the default when I last looked -- but that was quite some time ago, and I don't know about the current options/defaults for common X11-based display systems of today. Can anyone shed some light on this?
[...]
User connected to another machine through ssh (e.g. from a laptop to a
desktop). In this case DISPLAY is different for the client and for the
server process. (gui-get-selection) should be executed for client's DISPLAY.
In the past, I have regularly seen both varieties of this: where the user was logged into two machines, with an emacs server running on one, X displays on both, and wanted a new emacs frame on either one, depending on circumstances. As near as I can tell, there is no "right" choice here; the user needs to (be able to) provide input on which display to use.
This is a fundamentally different case from "the user wants to use a non-graphical X11-based mechanism that emacs normally doesn't enable until emacs actually connects to a working X11 display", and I think it should be possible to get emacs to make that connection without necessarily popping up a window that the user doesn't want.
I hope that helps,
~Chad
P.S. Now that I've said it "out loud", I expect to learn that either the Wayland people now consider that a security hole and will disallow it, or that they already did so.