I'm not sure just what the process was behind this question being closed. Honestly, it feels as if someone was either in a hurry to just close it or wasn't looking over everything, including the comments. Also, One important comment has been deleted.
I posted the question before going to sleep. I'm looking for a way to do a popup in Blender, with Python, that is modal (in other words - stops a script until I respond to it) and a popup. Note that's 2 main points: 1) Modal, 2) Popup. I woke up and there were two comments, one by @Alexbries, with 2 links that I looked over and another by @p2or, with another link. I looked over that, as well.
I responded to the first comment about those links not providing what I had hoped they would and what I thought was a problem with what I was trying to do. Then I looked at the link @p2or provided and found it wouldn't work. I explained why: The answer in that link did not provide a true popup, but a panel in the 3D VIEW space that (at least when I ran the code in the answer) would not show up unless you had the tabs pulled out and were looking at the right tab. No popup - which is an important part of the question.
Later I come back and find the question is closed because there is an answer already provided on another question. I go there, hoping for a good answer and it's the same answer that had been given in @p2or's comment, the one I said was not a full answer. So I was thinking, "Maybe the person who closed it didn't pay attention to everything here," and looked and that comment, with the link to that answer, was gone.
I feel a bit like I'm in a whack-a-mole game. I get a comment suggesting an answer and I check on it and leave a comment why it's not a suitable answer. Then I come back, find that comment (with the link) deleted. My reasoning why that answer wouldn't work is still there, in a comment, but someone has now closed the question - for the same reason given in that now deleted comment. BUT - my explanation would still apply, which means if the person closing the question had inspected the code in that answer AND my comment, they'd have seen the reason that answer doesn't work.
I'm trying to get an answer to this, even if it is, "You can't do this with a popup." I've gone through man SE:Blender questions and answers, through Blender forums, and still can't get a clear answer on this - and the answer some people think is an answer is not.
If someone knows of an answer that works and can point it out, I'm fine with the question being marked as a duplicate, but that's not happened and I don't want to have to keep repeating that this one answer does not answer the question.