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
* This method should be called (by the content provider) when elements have
	 * been removed from the model, in order to cause the viewer to accurately
	 * reflect the model. This method only affects the viewer, not the model.
	 * </p>
	 * 
	 * @param elementsOrTreePaths
	 *            the elements to remove
	 */
	public void remove(final Object[] elementsOrTreePaths) {
		assertElementsNotNull(elementsOrTreePaths);
		if (elementsOrTreePaths.length == 0) {
			return;
		}
		preservingSelection(new Runnable() {
			public void run() {
				internalRemove(elementsOrTreePaths);
			}
		});
	}
	/**
	 * Removes the given elements from this viewer whenever they appear as
	 * children of the given parent element. If the given elements also appear
	 * as children of some other parent, the other parent will remain unchanged.
	 * The selection is updated if required.
	 * <p>
	 * This method should be called (by the content provider) when elements have
	 * been removed from the model, in order to cause the viewer to accurately
	 * reflect the model. This method only affects the viewer, not the model.
	 * </p>
	 *