> It blocks the system with a demanded action
Yes, in almost every case, the default "popup" GUI library call is also a modal dialog. You cannot access anything else, anywhere else, in the entire program (even if the program had multiple separate windows open). All you can do is read the dialog's text, and hit the "ok" button to dismiss it (or pick from one of a set of "buttons" that are shown on the popup to dismiss it).
The worst ones also do a global grab, with the result that you can't even switch away to another unrelated application on the system without first "interacting" with and dismissing the popup.
To see the version that is built into Javascript in the browser, put the following into the URL field of a new bookmark, and save the new bookmark.
javascript:(function(){alert("hello");})();
Then, while here on HN (or anywhere else), click that new bookmark you just made, which will pop up the default built in Javascript alert box, and try to interact with the rest of the page it pops up in front of.