🐛 amp-autocomplete: Fetch remote data after first user interaction

[caroqliu]

Resolves #23772

Currently amp-autocomplete will fetch remote data source on layout. This change:

• delays this initial fetch until the user first interacts with the input field contained in the component
• does not impact behavior for data provided inline

[sparhami]

I think this makes sense as the default behavior, but it might be nice to support prefetching of the data (via an attribute) to improve responsiveness for users.

I would also suggest looking into using touchstart for prefetching of the data (in addition to starting the fetch if you have not already on focus). The focus event does not occur until touchend (or mouseup), which can occur hundreds of milliseconds after touchstart due to the human latencies involved of touching and releasing.

The trade-off is that you start prefetching results when the user might have been scrolling, and happened to swipe on the input during their scroll. Since many auto completes are used as search bars (near the top of the page), users are unlikely to scroll on that part of the screen, so I'm not sure that this is a huge downside.

[caroqliu]

Thanks for the comments! Filed #24687 as follow up to support prefetching through an attribute. Also added listening on touchstart since it is unlikely for scrolling to occur on this element. cc @sparhami