Interface ViewerBeforeDownloadDetail

What the viewer hands the element of the handler showing a file, with the before-download event it dispatches on it before downloading that file, from its own Download, Ctrl+S or the Files download action. A handler with edits not written yet saves them first:

element.addEventListener('before-download', (event) => {
  if (dirty) {
    event.detail.waitUntil(save())
  }
})
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A promise that rejects cancels the download, and the viewer says so.