Q2W3 Fixed Widget for WordPress WordPress plugin - Plugin for that
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How to Add a Floating Sticky Sidebar Widget in WordPress | WPism
How to Create a Sticky Floating Sidebar Widget in WordPress
Q2W3 Fixed Widget (Sticky Widget) - Fixes positioning of the selected widgets, when the page is scrolled down. #WordPress | Widget, Tag cloud, Cloud template
How to make your WordPress Sidebar (or widgets) 'Sticky'
How to Set a Fixed Widget Position in WordPress
Fixed Widget and Sticky Elements for WordPress – WordPress plugin | WordPress.org