I suspect you could be both right and wrong either way.
If we accept "scrollbar" then that means we have added a new word to the (English ?) language. And someone is sure to complain that we no longer speak like Shakespeare.
If you use "scroll bar" you are referring to the bar and qualifying it as type 'scroll', 'scroll' is an adjective. And we do say it like that still IHMO.
Personally, I think a scrollbar is a thing in its own right, it deserves its own name. So, I would leave out the space. But I am an engineer and scientist, not skills with the English Language at all !
The Oxford puts a dash in there, scroll-bar; Collins, Webster and Cambridge say Scroll bar; Wikipedia scrollbar. Like I said, you will offend someone how ever you do it !
Davo