This feature is deprecated.
On WordPress v5.6 jQuery was updated to a new version and Smooth Transition doesn’t work correctly.
The Smooth Transition widget allows you to manage the animated transition between pages in a gentle way. Your transition effect will improve the user experience.
Moreover, by choosing the # content as a selector, the header and footer will not change and the transition between the pages will be more intuitive.
You can apply the smooth transition to all the pages of your website.