Answer
The screenshot is captured before the introduction opens and stays in memory until you choose Report an Issue. The form then shows its preview and starts uploading. Closing the introduction discards it without uploading. If capture fails, no automatic screenshot is attached.
Review the capture before submitting. It may include visible messages or personal information, and it can omit the keyboard, system overlays or native media. Remove it with its thumbnail X or retry a failed upload. It counts toward the ten-image limit; a recovered ten-image draft keeps its existing attachments.
Shake to report an issue starts enabled and can be turned off in the introduction for that device. It works only while signed in and foregrounded; motion readings are not uploaded. Browsers use the account menu and do not capture automatically. Opening from the menu also does not capture. Installed apps need the supporting update; a web release cannot update them.