clientY vs pageY

A point in a given page ,clientY yes pageY The distance from this point to the viewport and the top of the page （ In pixels ）.

ad locum , page Is an entire rendered page that may have scroll bars , and viewport Is the visible part of the page .

┌─────────────────────────────┬─┐︎─── Web page      ▲
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|                             | |                 pageY
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├=============================┼─┤       ▲           |           ▲
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