scroll-bar-adjust-thumb-portion

scroll-bar-adjust-thumb-portion is a variable defined in `C source code'.
Its value is

t

Documentation:
Adjust thumb for overscrolling for Gtk+ and MOTIF.
Non-nil means adjust the thumb in the scroll bar so it can be dragged downwards
even if the end of the buffer is shown (i.e. overscrolling).
Set to nil if you want the thumb to be at the bottom when the end of the buffer
is shown. Also, the thumb fills the whole scroll bar when the entire buffer
is visible. In this case you can not overscroll.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.4 of Emacs.