Scroll constraints should not allot full scroll extent if size does not require it

[stargazing-dino]

I currently allow stuff that pass me a scroll controller to go past the immediate viewport into extentBefore and extentAfter. HOWEVER. If it's something that chooses to expand as large as possible such as a Center then we should try constraining the child size first to the viewport and then, if it fails, fallback to giving it all the possible size.