- add a new EntityPrototypeFlag: "allow-gui". Adding this flag to an entity prototype makes Entities fire the "on_gui_opened" event when clicked.
- in any on_gui_opened event that is fired by an entity that has no "natural" GUI, the gui_type in the event is set to defines.gui_type.none.
This would allow adding custom GUIs to any entity that does not have a natural GUI. A mod would listen for that event and set player.opened accordingly to the custom GUI.
The only way right now to achieve this is by adding a custom event that reacts on any mouse button press, filter and only open the custom gui if the pointer is over the entity. This is cumbersome and probably not very efficient. Or one has to find a matching entity with a GUI and (ab-)use that for the custom entity.
(My hunch is that this touches on some internal hardwired logic and can not be implemented for that reason. But I thought that I could ask.
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