clientY

Gets the distance in pixels between the pointer and the top edge of the browser's viewable area.

The browser's viewable area refers to the viewable contents of the page and excludes elements such as a toolbar and scrollbar. Therefore, two clicks on the same physical point of the screen return the same clientY values if the page is scrolled vertically in the meantime. But two clicks on the same point on the page return different clientY values if the page is scrolled vertically in the meantime.