Paint the first candidate that resolves to a displayable image as target's background,
skipping any that do not. A verify candidate is HEAD-checked first, because YouTube
answers a thumbnail a video does not have with a valid placeholder image under a 404,
so its load event alone cannot tell the difference.
Candidates are probed concurrently to overlap the network, but consumed in priority
order, so a fast low-quality thumbnail cannot beat a slower preferred one.
Paint the first candidate that resolves to a displayable image as
target's background, skipping any that do not. Averifycandidate is HEAD-checked first, because YouTube answers a thumbnail a video does not have with a valid placeholder image under a 404, so its load event alone cannot tell the difference.Candidates are probed concurrently to overlap the network, but consumed in priority order, so a fast low-quality thumbnail cannot beat a slower preferred one.