popupalign is an optional attribute for specifying which side of the popup content should be attached to the popupanchor.
<element popupalign="none | topleft | topright | bottomleft | bottomright" />
<element id="edit-context" popup="editor-popup" popupanchor="topleft" popupalign="bottomright" />
The popupalign attribute can be used to specify which corner of the popup content is tied to the originating point. If omitted, the default is topleft. The example below shows how to create the traditional buttons with attached left mouse menus that exist in the 4.x Communicator product. If omitted, no anchoring occurs. Instead the popup content comes up directly underneath the mouse event coordinates. This point (either directly under the mouse or attached to one of the four corners) is called the originating point.
By default the popup content appears with its top left point located directly underneath the point at which the user's mouse goes down (on tooltips the content is displaced by the height of the mouse cursor). The default value for both popupanchor and popupalign is "none."