I _think_ what's happening here is that the content is handling the pinch rather than APZ, and `preventDefault()`ing some but not all of the touch events. The remaining touch events look to the APZ gesture detector like a long-tap, and trigger a context menu event.
Bug 1570559 Comment 2 Edit History
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I _think_ what's happening here is that the content is handling the pinch rather than APZ, and `preventDefault()`ing most but not all of the touch events. The remaining touch events look to the APZ gesture detector like a long-tap, and trigger a context menu event.