OK it seems that when the height of the scrollbar is being determined in the function "updateThumb" in ScrollBar.as, it uses the variable _pageSize, which contains the number of lines that a page contains.
If the TLFTextfield is measuring _maxScrollPosition in pixels then for the thumb to scale properly, _pageSize should also be measured in pixels. But it´s not. Infact it always seems to contain the default 10 value.
So I extended, UIScrollBar.as and overrode "updateThumb" replacing _pageSize with _scrollTarget.height and everything works as it should now.
Hope this helps someone