Kai Grossjohann
"My" popup wasn't displayed at 0,0; it was displayed near the center (but a little bit east and south of it) of the primary head.
FWIW, if I move the popup to the right workspace (and thus head) once manually, then subsequent popups show up on the right head. (Or do they show up in the right workspace? I didn't try to switch workspaces between invoking the popup and the window appearing.)
I'd guess that the program's remembering the position, which would make lots of sense. So that's two cases: the program doesn't care, and says (0,0) the program thinks it knows, and says (1487,283) (say) In the second case I guess it wouldn't be unreasonable for a window manager to calculate the relative position on the screen and move it appropriately. Feels risky but I guess I'm thinking of things like panels and gkrellm and things, but they're not transient windows (they're just treated much the same as floating windows). That leaves a possible class of programs that specify an absolute position and really mean it. I can't think of any plausible examples, but I'd guess it's conceivable. I've got one application that tries to open a transient in the center of the screen but doesn't know about Xinerama (so that fails on my 2-screen setup). So maybe the second case above (where the application specifies a position) could look to see if the requested window would be overlapping two screens, and if so could move it?