Just to put in my 2 cents: Hiding scrollbars is just a bad idea IMO. Sure, it looks neat. But why do we need to have form over function in a UI? There is ample space to be claimed by a scrollbar. I immediately see how much content there is, and that there is more content in the first place. I can easily, in a fraction of a second, scroll to line 3,500,000. I use the scroll wheel for small amounts of scrolling, only. Anything that takes longer than a second starts to become very annoying.
I dislike smart phone UX where I have to scroll dozens of time to get where I want to be, even though there is actually more reason to hide them on those screens because of space constraints. But why not show them when you start scrolling, and make them controllable by touch then? Some software actually does it that way, and it makes using them much easier and faster.
There is simply no good reason (apart from looks) why you would want to hide scrollbars if you have the space to show them. It gives you zero advantages, and plenty disadvantages.
Unless you port Haiku on small screen devices, and provide a patch for the option of hiding them then, I’m pretty sure this won’t happen in Haiku.