At least in the case of Dear ImGui, things like the current window position and size is widget state that's owned and persisted between frames by the library, otherwise windows wouldn't be moveable or resizeable by the UI user.
Other immediate mode UIs may decide to delegate this "state housekeeping" to the user (that's how rxi's microui works), but IMHO this isn't quite as convenient for the library user, it keeps the library implementation very simple though.
Other immediate mode UIs may decide to delegate this "state housekeeping" to the user (that's how rxi's microui works), but IMHO this isn't quite as convenient for the library user, it keeps the library implementation very simple though.