Popup does not re-position when MainWindow is moving.

[zenjia]

When the Popup is opened, it will not reposition according to the reposition of the MainWindow. To reposition it correctly, I've to subscribe to the PositionChanged event of MainWindow, which is not so elegant. Is this behavior by design? Or is there a better way to reposition the Popup?

[EDIT]
I've test WPF and UWP: WPF has the same problem/behavior, while UWP doesn't. To test, just open a popup and let StaysOpen = true(or IsLightDismissEnabled = false), then drag the MainWindow.