How do I get the Photopea API to work as described?

According to the documentation for the Photopea API, the “script” parameter of the JSON configuration object “should be executed after loading each file (can be long)”

However, when I test the following simple JSON object within the Playground page, it appears that the script is trying to run BEFORE the files have fully loaded:

{
    "files": [ 
        "https://www.photopea.com/api/img2/pug.png",
        "https://www.photopea.com/api/img2/pug.png"
    ],
    "environment": {
        
    },
    "script" : "app.documents[1].rotateCanvas(90);"
}

The script attempts to rotate the canvas of the 2nd document, which presumably fails because the script executes before the 2nd file has loaded.

How, using the API, can I get the script to run only AFTER all the documents have loaded?