Bug 81877 Package explorer forgets active working set [package explorer]
3.1 M4 1. open pkg explorer. 2. select a working set, pkg explorer updates and filters out projects. 3. close pkg explorer. 4. open package explorer again and the working set is no longer applied. This used to work.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/packageview/PackageExplorerPart.java
object = filter(object, parent, filters);
					if (object != null) {
						list.add(object);
					}
				} else
					list.add(object);
			}
			return list.toArray();
		}
		/*
		 * @see AbstractTreeViewer#isExpandable(java.lang.Object)
		 */
		public boolean isExpandable(Object parent) {
			if (isFlatLayout())
				return super.isExpandable(parent);
			if (parent instanceof IJavaProject) 
				return super.isExpandable(parent);
			
			ViewerFilter[] filters= fViewer.getFilters();
			Object[] children= ((ITreeContentProvider) fViewer.getContentProvider()).getChildren(parent);
			for (int i = 0; i < children.length; i++) {
				Object object= children[i];
				
				if (isEssential(object))
					return true;
				
				object= filter(object, parent, filters);
				if (object != null)
					return true;
			}