Apply pinch zoom deltas to the compositor's visual viewport transform
so the currently presented display list can respond without waiting for
the WebContent main thread. Keep the normal WebContent pinch event path
so the real VisualViewport state and DOM-visible events catch up after.
Only take the compositor path when async scrolling is enabled and there
are no blocking wheel listeners, since pinch zoom dispatches a synthetic
wheel event that script may cancel. Coalesce queued pinch events in
WebContent so main-thread catch-up can adopt multiple gesture deltas
together.
Use the compositor visual viewport transform for wheel hit testing and
consume wheel deltas as visual viewport pan while zoomed. Scale the
handoff to layout viewport scrolling by the inverse visual viewport
scale, so touchpad momentum does not jump when the visual viewport hits
an edge.
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