Defers costly content (a decoded image, a formula bitmap, highlighted code, a diagram) until the element that shows it is about to scroll into view, so the parts of a long document nobody reaches cost nothing. Renderers honour IsLazyRenderingEnabled before using it.
public static class ViewportTracking
kind:method, kind:property, kind:ctor. Press Esc to clear.RunWhenNearViewport(Control, Action)Calls realize once, the first time element is laid out within one viewport height of the visible part of its scroll viewer (or of its window, outside one). It is called during the layout pass that brings the element near, before that pass is rendered, so content it builds is laid out before it is ever seen. Call it before the element is attached to a visual tree, as a renderer does.