I'm not too clear as to what exactly you are trying to achieve. If you run, say, an automation plugin's GUI, it should be modal, in which case there is no advantage to have a separate process. If you are trying to create something semi-modal where the user can interact with Photoshop AND with your GUI simultaneously, the only official supported way to do it is to use CSXS and have your GUI in an extension panel. Everything else (i.e. some sort of fake semi-modal dialogs ran by a separate thread or another process) is a hack, they are explicitly unsupported by Adobe and will require tinkering with Photoshop's internal event look, explicit maintenance of Z-order, substituion of windows callbacks etc. While its doable, maintaining this sort of semi-modality is not a trivial task, it will likely to break from one release to another and you will be more or less on your own with it, since there is no simple solution for it. Just something to consider.
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