When an amp-list refetches its data due to the src changing via amp-bind or the soon-to-be-implemented refresh action being triggered (#13717), the amp-list just shows the previous data until the new data is returned by the server. This can create a weird user experience in lots of cases, because the user taps some button which should change the data but the page seems to not react at all until the refetch is complete. I think it would be better to instead hide the previous data during the refetch and show the loading indicator and placeholder while it is loading to make it clear that the data is being refreshed.

cc @choumx @aghassemi