by monetizemore on WordPress.org

A fixed widget and a stop widget in action
Use Fixed Widget to create sticky widgets, sticky blocks, and other elements that stay in the visible screen area when a user scrolls the page up or down.
Sticky widgets are more visible than unfixed widgets and therefore have a significantly higher click-through rate.
That’s why this option is worthwhile for ads or other elements that visitors should interact with. Meanwhile, Google also allows the integration of sticky AdSense ads.
Fixed Widget is completely free of charge.
Theme requirements:
wp_head() and wp_footer() functions in header.php and footer.php files