If there are no natural or calculated height and width values, these dimension values are mathematically NaN (Not A Number). The elements are waiting for their layout container to give them dimensions. After layout is run, there will be values for ActualHeight and ActualWidth properties for elements where a Stretch alignment was used. The NaNvalues remain in Height and Width for the child elements so that the adaptive behavior can run again, for example, if layout-related changes such as app window sizing causes another layout cycle.