An activation that re-showed a fallback while a retry was in flight armed a
second timer, so two retries could run at once and land out of order, leaving
the cache and the screen on different providers. Retries now run one at a
time; a request made meanwhile re-arms once the current one ends, and a
result that is obsolete or no longer wanted is disposed.
Imagery and terrain retries are now handled as each settles, so a stalled
terrain attempt no longer keeps recovered imagery off screen, or the reverse.
