magecoreui/stickyfloat
This plugin makes it possible to have a fixed position element that is relative to it’s parent. A normal fixed positioned element would be “out of context” and is very difficult to use in the most common situations with fluid designs. This plugin solves that problem with as little code as I could possible get it so it will run in the most optimized way, while also allow you to customize it in many important ways which might suit you best.