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
TransferDragSourceListener[] dragListeners= new TransferDragSourceListener[] {
			new SelectionTransferDragAdapter(fViewer),
			new ResourceTransferDragAdapter(fViewer),
			new FileTransferDragAdapter(fViewer)
		};
		fViewer.addDragSupport(ops, transfers, new JdtViewerDragAdapter(fViewer, dragListeners));
	}
	private void initDrop() {
		int ops= DND.DROP_COPY | DND.DROP_MOVE | DND.DROP_LINK | DND.DROP_DEFAULT;
		Transfer[] transfers= new Transfer[] {
			LocalSelectionTransfer.getInstance(), 
			FileTransfer.getInstance()};
		TransferDropTargetListener[] dropListeners= new TransferDropTargetListener[] {
			new SelectionTransferDropAdapter(fViewer),
			new FileTransferDropAdapter(fViewer),
			new WorkingSetDropAdapter(this)
		};
		fViewer.addDropSupport(ops, transfers, new DelegatingDropAdapter(dropListeners));
	}
	/**
	 * Handles selection changed in viewer.
	 * Updates global actions.
	 * Links to editor (if option enabled)
	 */
	private void handleSelectionChanged(SelectionChangedEvent event) {
		IStructuredSelection selection= (IStructuredSelection) event.getSelection();
		fActionSet.handleSelectionChanged(event);
		if (isLinkingEnabled())
			linkToEditor(selection);
	}