This issue can be seen in the SampleBrowser, SyntaxEditor GettingStarted Examples 10 and 11.
As an image is more meaningful, have a look at this screenshot showing that the completion list is misarranged due to not being closed on moving/resizing application window. Same is true for the parameter info tooltip.
In these cases, the syntax editor will not lose focus or keyboard focus. Obviously, other notifications, like SelectionChanged and similar, are not called either. For that reason, the completion list will not be automatically closed. Furthermore, I can't cancel auto complete manually.
Only way I yet figured out is hooking to any events which will occur in these instances, i.e. OnSizeChanged and OnMouseDown events of the root visual element. This, however, is rather a workaround than a true fix.