Hi @Blaceus, Clientside JavaScript does not communicate with the Python server (and thus the rest of your callbacks) unless you explicitly define a Clientside callback (see an example here).
How I see your issue solved is this: instead of an eventListener, you define a Clientside callback with Input your activation Button and Output("iframe-message", "children"). Then instead of setting the innerHtml in your JavaScript you simply return the text from the iFrame.
Then you can catch the update of the children property of your "iframe-message" div in a regular Python callback.