Bug 43035 Smart Tab ends up at offset 0
Have a line that has an indent greater than it is required. Place cursor at the correct indent position. Press Tab. => Cursor ends up at the beginning of the document.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/actions/IndentAction.java
* @return the editor's selection provider or <code>null</code>
	 */
	private ISelectionProvider getSelectionProvider() {
		ITextEditor editor= getTextEditor();
		if (editor != null) {
			return editor.getSelectionProvider();
		}
		return null;
	}
	
	/*
	 * @see org.eclipse.ui.texteditor.IUpdate#update()
	 */
	public void update() {
		super.update();
		
		if (isEnabled())
			setEnabled(!getSelection().isEmpty());
	}
	
	/**
	 * Enablement when tab key is pressed - the current selection has to be empty and in the
	 * whitespace in the beginning of a line.
	 */
	protected void updateForTab() {
		super.update();
		
		if (isEnabled())
			setEnabled(isSmartMode() && isValidSelection());
	}