> If I'm reading it completely, glancing at the scroll bar tells me where I am in the document so it tells me if the document is reaching its conclusion, or perhaps just going on a side note.
You'll see the hidden scrollbars when you're reading through something anyway.
> A bit later, when I have read a page or two, how far have I advanced?
You'll see the hidden scrollbars as you scroll through the pages to begin with.
> do you have an explicit implementation in mind where you can optionally enable scroll bars? I don't think I've ever seen such an option, and certainly not easily accessible.
It's right there in Mac general preferences, defaulting to visible if you use a non-Apple mouse.
You'll see the hidden scrollbars when you're reading through something anyway.
> A bit later, when I have read a page or two, how far have I advanced?
You'll see the hidden scrollbars as you scroll through the pages to begin with.
> do you have an explicit implementation in mind where you can optionally enable scroll bars? I don't think I've ever seen such an option, and certainly not easily accessible.
It's right there in Mac general preferences, defaulting to visible if you use a non-Apple mouse.