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
public void partOpened(IWorkbenchPart part) {
		}
	};
	
	private ITreeViewerListener fExpansionListener= new ITreeViewerListener() {
		public void treeCollapsed(TreeExpansionEvent event) {
		}
		
		public void treeExpanded(TreeExpansionEvent event) {
			Object element= event.getElement();
			if (element instanceof ICompilationUnit || 
				element instanceof IClassFile)
				expandMainType(element);
		}
	};
	private class PackageExplorerProblemTreeViewer extends ProblemTreeViewer {
		 
		List fPendingGetChildren;
		
		public PackageExplorerProblemTreeViewer(Composite parent, int style) {
			super(parent, style);
			fPendingGetChildren= Collections.synchronizedList(new ArrayList());
		}
		public void add(Object parentElement, Object[] childElements) {
			if (fPendingGetChildren.contains(parentElement)) 
				return;
			
			
			List l= (List)fAdditionalMappings.get(parentElement);
			if (l == null) {