If you’ve designed a project with an inclined ceiling (How to insert an inclined ceiling) and you open the virtual navigation window, the windows in your project may show a gray object which cuts through them as seen below:
Close the virtual navigation window and any 2D wall or floor windows open.
Click the button from the Ribbon and open the Navigation and Rendering tab. Deactivate the “View external walls” flag and close the Options window.
Reopen the virtual navigation window and the windows will now be displayed correctly.
This article is valid for the 2018 version and later.