Created attachment 80176 [details]
Calculated glue point

Open attached file. It has a "non-primitive" custom shape triangle. The upper point is set by a handle and can be moved horizontally. There are two glue points. On is fixed on the lower edge of the shape, the other is calculated so that it has the same position as the upper point. This two glue points are connected with a straight connector.
Move the handle right and left and notice, that the glue point and the connector follow the movement of the handle.
Now drag the handle far to the right, leaving the viewBox. You see the path of the shape follows the movement, but the glue point stops at the right edge.

The glue point follows the handle always in PowerPoint 2013. If find no place in the spec, where it says, that glue points are restricted to the viewBox. Therefore I think it is a bug.