static public IEnumerable <KeyValuePair <IUIElement, RectInt[]> > GetOccludingUIElementAndRemainingRegion(
this IUIElement occludedElement,
object uiTree)
=>
occludedElement.GetUpmostUIElementOfSubtreeInFront(uiTree)
// Assume that children of OccludedElement do not participate in Occlusion
?.Where(candidateOccluding => (occludedElement?.ChildLastInTreeIndex ?? 0) < candidateOccluding?.InTreeIndex)
?.Select(occludingElement => new KeyValuePair <IUIElement, RectInt[]>(
occludingElement, occludedElement.Region.SubstractionRemainder(occludingElement.Region).ToArray()))
// only take elements where the remaining region is smaller than the region of the OccludedElement.
?.Where(occludingElementAndRemainingRegion =>
(occludingElementAndRemainingRegion.Value?.Select(subregion => subregion.Area())?.Sum() ?? 0) < occludedElement.Region.Area());
