FormOverlay centers overlay off the page

FormOverlay calls centered() whenever a visibleChange event occurs. This causes overlays that are taller than the viewport to be positioned where the top of the overlay is off the page. The simplest solution is to check whether bounding_box.get('offsetTop') is less than zero and then set it with bounding_box.setStyle('top', X).

Another solution would be to use widget.align(null, [WidgetPostionExt.TC, WidgetPositionExt.TC]) instead of widget.centered(), but if the window was really big, the form overlay would pop up a long way from where the cursor clicked to activate it.

Instead of centering the widget vertically, it would be convenient to position the top of the widget near the element that was clicked on to show the widget.