With the DropDownList control we split up the two functionalities - detecting the edge of the screen, and choosing the direction based on the available space. In the version you are using the properties are already implemented:
We chose to leave the DirectionDetection disabled by default so that we don't break the existing behavior for people using the DropDownList.
Try setting the EnableDirectionDetection to true for the DropDownList in your template and it should work as expected.
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