I'll do my best to explain what I'm trying to do here:

I am creating a Google apps script add-on which has a sidebar interface. When the user clicks one of the buttons, it will load the Google File Picker for them to upload a file. Once the file is uploaded in the picker, the picker window is set to close automatically and then some background functions run (on the server side) to import the data from the selected file.

On the original sidebar I have a status bar at the bottom which shows if the add-on is still running or not (still importing the data). The problem is I can't update the status bar element to say it's done because it's a separate window (the picker window) that imported the file and then ran the server side functions.

I'm wondering if there is a way to have an event listener in the sidebar that checks to see if the picker window has been closed (after it runs the server side functions) so that it can then update the status bar to say "complete"?

I haven't been able to find anything about having an event listener that monitors another window.

Suggestions for another way of approaching this completely are welcome as well.

There is a callback function for the file picker window once a file has been uploaded. However, I'm not aware of a way to use that to write HTML on another window (the status bar to say "complete)