With BUFFER_LONG = 1, refetch threshold was halfLong - 1 = 11,
which meant each refetch shifted the corridor centre by ~11 tiles.
From the camera's view that looked like the corridor suddenly
jumping 11 tiles forward — described as 'tiles jump to the right'.
BUFFER_LONG = 11 -> threshold = 1, so refetch fires as soon as
the camera crosses one tile beyond centre. Each refetch shifts by
~1 tile, which the user perceives as a smooth scroll rather than
a jump. Refetches now happen every 8 sec instead of every 92 sec,
but the load is small (only ~1 net new tile since browser caches
ESRI responses for 24h).
Richard Zink
faabb44cdb