Vignette ads now support a larger range of screen sizes. For sites that have vignettes turned on, vignette ads will soon start to appear on wider screens such as desktop. Our experiments show that vignette ads perform well on wider screens.
If you'd prefer not to show vignettes on desktop, you can turn this option off using the new "Wide screen" control in your Auto ads settings. Note that wide screen vignettes won't start serving until after November 3, 2020.