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Viewport tab specify viewport settings for ATF SDK, which are used for example by Infinite Scroll feature.
Viewport offset top [in pixels] - offset from top of the viewport which is considered as part in the viewport
Viewport offset bottom [in pixels] - offset from bottom of the viewport which is considered as part in the viewport
Visibillity Threshold stands for the ratio of element height which determines when the element can be considered as visible in the viewport. Threshold of 30% requires 30% of the element height to be in viewport to be considered as visible