[ anchor : handle ]A handle to a DOM node in a page to which the panel should appear to be anchored. If not given, the panel is centered inside the most recent browser window. Note that it is not currently possible to anchor panels in this way using only the high level APIs.
You need to instead create your panel, then assign it to the panel
property of the Widget:
[ panel : Panel ]
An optional panel to open when the user clicks on the widget.
Note: If you also register a "click" listener, it will be
called instead of the panel being opened. However, you can show
the panel from the listener by calling this.panel.show().
See this SOF comment for an example: