multiseat clicks

[martinetd]

Two things I found running sway but probably happen with rootston as well:

• for xwayland, moving a cursor in seat0 then clicking on mouse in seat1 without moving will send click to where seat0's mouse was. I know xwayland doesn't understand seats so it might be normal, but I think we can send the new position just before sending the click to work around that.
• for native wayland (gnome-terminal), clicking on mouse in seat0, holding click, then moving mouse in seat1 will move selection around as if seat0's mouse was moving.

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wlroots has migrated to gitlab.freedesktop.org. This issue has been moved to:

https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/566