It is possible to move windows partly or fully out of view. After doing a page-reload, the window is in view again.
I think windows should always jump back when they were put out of view, so that at least a part of the header is visible again (the part where you can move the window).
Edit:
Another situation is the following:
The Browser Window has a view-height of 400px, a vaadin-window with a height of 500px is being opened and centered; now the head of the window is out of view so it cannot be moved until the browser window is enlarged and reloaded, but this isn’t always possible (e.g. when using a smartphone).