I have a parent with position:fixed; width/height:100%; overflow:auto and a child that also has position:fixed. When the parent's content overflows and the parent starts scrolling I would expect the child to stay fixed in relation to the viewport. It does not.
the scenario is a slide down panel on top of the content using css transforms. The slide-down panel has a close button that should stay in view while the panel's content scrolls. Change the .overlay-slidedown class on the .overlay element to .overlay-fade and, in that case, the close button will stay in it's place.