The dropdown is
scrolled with the page.
Now, there is this other behaviour, when the combobox is within a scrollable container. In that case the when you scroll the container, the combobox dropdown will not scroll. If this is the case you are referring to -- this is the default behaviour and can not be changed out of the box. We'll be adding however a help article how to change the behaviour.
When we are ready with with the article it will be available in our help, and we'll link to it form the common combobox resources. I will also reply here.
Note: the script will need some tweaking.
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