The problem you described is due to the fact that all the DockWindows excluding the first displayed in the tabstrip are not yet Visible and have empty sizes. Once you navigate all tabs, the Preview will work fine.
We will address this issue in two ways:
- Add property in the QuickNavigatorSettings, named ForceSnapshot, which, if true, will update the size and visibility of the Window, so that a correct snapshot may be taken. This will bring some additional overhead and that is why we will allow our users to specify whether this feature is desired or not. It will be true by default.
- Add event, that will allow users to specify the completely custom image as a preview.
the Telerik team