I'm having issues with the behaviour of the DropDownList in one of our applications.
The DropDownList sticks to the Input/Select-field where you opened it from while scrolling the page
The DropDownList moves with the whole page, as the page is scrolled.
I can show you an example in the gif which is in the attachments.
My guess for this is the CSS-property "top" which makes the dropdown move while scrolling, but since the top-property gets set by your controls (I think) its hard to get a working solution.
I also checked the CSS-propertys of your Demo:
I checked my own and your demo-example with the devtools and the CSS looks exactly alike.