On touchscreen laptops (e.g. Surface), scrolling a fixed-layout book
webtoon-style with two fingers moving the same direction accidentally
triggered pinch-zoom. The old code committed to a pinch on the first
two-finger touch and applied the raw distance ratio from the first move,
so a slightly non-parallel scroll drifted the finger spacing and zoomed.
Defer the decision with a pending state: on two fingers, compare the
change in finger separation against the midpoint travel. A pinch changes
separation while the midpoint stays put; a scroll moves the midpoint
while separation barely shifts. Zoom only engages once separation change
crosses a 24px deadzone and outweighs the pan distance; a 12px pan locks
the gesture as a scroll and lets the page scroll natively. On pinch
confirm, re-baseline the distance so zoom starts at 1x with no snap.
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Huang Xin
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