Any way to know when the screenshot process starts/finishes?

Sometimes there's a few seconds delay between pressing the button and when the button becomes disabled. This causes confusion to the end user. I would like to have some sort of signal from {capture} -- whether it's a javascript event that gets triggered or a Shiny input or anything else -- that notifies me when the button is first pressed and when the download begins. That way I could let the user know when they're waiting.

Start by tracing the screenshot button flow in {capture}, from the initial press through the point where the download begins. Determine how the project currently handles the delay and identify the existing entry points involved; done means callers can observe both lifecycle moments and use them to show waiting feedback.

Written by the indexing model from the issue text.