Bug 161086 [Viewers] performance problem in AbstractTreeViewer.internalExpandToLevel
If you expand to ALL_LEVELS, internalExpandToLevel will call createChildren even if the element has no children, i.e. it does not check if the element is expandable. Found this while experimenting with the Quick Access feature (enter "con", then delete all three characters one at a time).
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/AbstractTreeViewer.java
* the subtree rooted at the given element to the given level.
	 * 
	 * @param elementOrTreePath
	 *            the element
	 * @param level
	 *            non-negative level, or <code>ALL_LEVELS</code> to expand all
	 *            levels of the tree
	 */
	public void expandToLevel(Object elementOrTreePath, int level) {
		Widget w = internalExpand(elementOrTreePath, true);
		if (w != null) {
			internalExpandToLevel(w, level);
		}
	}
	/**
	 * Fires a tree collapsed event. Only listeners registered at the time this
	 * method is called are notified.
	 * 
	 * @param event
	 *            the tree expansion event
	 * @see ITreeViewerListener#treeCollapsed
	 */
	protected void fireTreeCollapsed(final TreeExpansionEvent event) {
		Object[] listeners = treeListeners.getListeners();
		for (int i = 0; i < listeners.length; ++i) {
			final ITreeViewerListener l = (ITreeViewerListener) listeners[i];
			SafeRunnable.run(new SafeRunnable() {
				public void run() {
					l.treeCollapsed(event);
				}