clientX

Gets the distance in pixels between the pointer and the left 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 sidebar and scrollbar. Therefore, two clicks on the same physical point of the screen return the same clientX values even if the page is scrolled horizontally in the meantime. But two clicks on the same point on the page return different clientX values if the page is scrolled horizontally in the meantime.