* We use a counting resource set, because elements might get removed
* from the set after they have been added again, e.g. when moving the
* mouse from over a resource item with popup to over a resource set and
* the popup removes the resource a bit later.
*/
highlightedSingleResources = new CountingResourceSet();
combinedHighlightedResources = new UnionResourceSet(
new DefaultResourceSet());
combinedHighlightedResources
.addResourceSet(highlightedResourceSetContainer);