It still seems that when there is no surfaces in a given area on a Weston display (when the desktop shell crashes), it doesn't clean the screen, which allows users to 'paint' over areas of the screen with a surface. Another issue with this is that it flickers aggressively as well when a window is moved.
This is not a bug that's what happens when there are no surfaces covering the screen in a given area. desktop-shell is responsible for providing that (the background image or a solid color image) and if that fails we get the behavior you describe. That behavior is not a bug, but any crash you encounter in desktop-shell is a bug as should be filed.
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