the Popup control always adapt his position so that it is fully visible on the screen. This works fine.
Now, if a visible Popup size changes, its position is not moved when it is not fully visible on the screen.

I've attached a test case for this issue:
Launch the test case.
Click on the "Popup" button : a Popup appears, it only contain a "Add Content" button.
Click on it several times, you'll reach a point where it is not fully visible: this is the issue.
Now, click anywhere to hide the Popup, and click again on the "Popup" button: the button position itself correctly.