Can you share the error messages that you get as well as your test configuration (browser, platform and selenium hub)? ATS is supposed to scroll to an element if it is not in the viewport before clicking. This might not be working as expected in the case with a popup that needs to be scrolled horizontally. However, what strikes me as odd is that FindWidget does not work. For FindWidget to work the element does not need to be in the viewport.
Probably ATS not being able.
But would question having a popup with a length scroll in the first place. If ATS cannot scroll; would a human user like to scroll?