Bug 135971 [CommonNavigator] Project Explorer: cannot drop to Hierarchy view
You can drag and drop an element onto the Hierarchy view from the Outline or Package Explorer. You cannot from the Project Explorer, you get a drop disabled cursor.
bundles/org.eclipse.ui.navigator/src/org/eclipse/ui/navigator/CommonViewer.java
protected void handleOpen(SelectionEvent event) {
		clearSelectionCache();
		super.handleOpen(event);
	}
	
	/* (non-Javadoc)
	 * @see org.eclipse.jface.viewers.StructuredViewer#handlePostSelect(org.eclipse.swt.events.SelectionEvent)
	 */
	protected void handlePostSelect(SelectionEvent e) {
		clearSelectionCache();
		super.handlePostSelect(e);
	}
	
	/* (non-Javadoc)
	 * @see org.eclipse.jface.viewers.StructuredViewer#handleSelect(org.eclipse.swt.events.SelectionEvent)
	 */
	protected void handleSelect(SelectionEvent event) {
		clearSelectionCache();
		super.handleSelect(event);
	}
	
	/**
	 * Clears the selection cache.
	 */
	private void clearSelectionCache() {
		cachedSelection = null;
	}
	
	/**
	 * Returns the current selection.