[reacocard]

Does it? If you can scroll further with the keyboard than the scrollbar can indicate, that would also make the scrollbar inaccurate. Perhaps instead we could add an indicator to the scrollbar (ala Chrome's text search indicators) that shows where the content actually ends.

[drey_]

Why not just make the scrollbar take the extra whitespace into account as if it's part of the webpage?

This is exactly what Scintilla (editing component) does if you enable that feature.