When use shadow element in listbox, the mechanism of render the list item is building a shell of item, create the list item content, finally filter out the shadow element. In function filterOutShadows(AbstractRenderer.java), after checkShadowElementAndCreateSubChildren, it builds out the component tree below the listitem and return items which is filtered out shadows. But after doing this, its retain unuseful shadow elements below the the host which waste the memory in server and unnecessary calculation in function checkShadowElementAndCreateSubChildren(AbstractRenderer.java)