When drawing a circular elipse, if one holds down the Shift key the Pattern Editor draws a "circle" as is expected in all drawing software.
(Photoshop, Illustrator, Paintshop, Corel, etc all have a hot-key for constraining an elipse to a circle when re-sizing)

After a shape is drawn in Pattern Editor, and you grab the re-sizing handles with the mouse, no combination of alt/shift/ctrl keys seem to work to keep the elipse constrained to a circular shape.

How does one constrain the elispe to a circle when re-sizing?

There is no hot key to keep circular shape when you resize it. You can open shape properties, use dimensions or calculate grid steps to ensure that the shape is circular after resizing.