During any drag longer than 120 ms the canvas rendered a full-resolution
frame every 128 ms under the finger. The settle timer was armed by the
first draft of a burst and not re-armed by later ones, so it counted from
the start of the gesture rather than from its last movement, fired
mid-drag, and the next coalesced event armed it again. Each of those
frames is the most expensive one the canvas draws, landing where the
frame budget is tightest.
The draft/sharp decision now lives in `refine::Refine`, apart from the
timers that carry it out. Every draft frame arms a settle timer carrying
a generation token and only the newest token is honoured, so the sharp
frame lands SETTLE_DELAY after the last movement. A request arriving
while a settle is still owed also counts as part of the gesture, so a
slow stretch of a drag (one event per frame, nothing to coalesce) no
longer renders sharp between drafts. A timer that fires with a render
already posted defers to it.
The tests drive the state machine through simulated timelines; the
long-drag case reproduced the four mid-drag sharp frames before the fix.
