Akiba So f48e43f13d fix(zoom): kill re-baseline pinch pop ...

Device log showed a single-frame scale pop (cur 0.504->0.694, a
+38% jump UP while the pinch was still shrinking).

Root cause: the absolute mapping targetScale = scaleStart *
details.scale is only valid when details.scale is 1.0 at the
moment scaleStart is captured. That holds at gesture start, but
on a mid-gesture re-baseline (a finger blips 2->1->2, routine on
Windows touch) a fresh scaleStart got multiplied by the
recognizer's still-cumulative details.scale, popping the zoom
then snapping back.

Fix: track rawScaleAtBaseline and normalize details.scale against
it so the cumulative reads 1.0 at every baseline. Extracted
absolutePinchScale() pure solver + 5 unit tests covering the
exact re-baseline scenario.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

2026-06-23 09:30:41 +08:00

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