If you want to continue showing "SAVING PROJECT FILE" message while the "Save as" dialog is on the screen, you have no other option but to switch from using "Browser invoke download" to "NWJS Show Save Dialog", and then "NWJS Write text file".
You can find how to use it in the official documentation, or search the forum for tutorials, examples. It's not difficult.
But I would just forget about it, and leave it as it is - invoke download and immediately unlock the UI in the same event.
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By the way, you should send the message to be shown in Text_Panel_CPU_Is_Busy in function parameter, and set text inside the function.