XmTextGetTopCharacter accesses the position of the text at the top
of the Text widget.
If there is no text in the Text widget (in other words,
XmNvalue contains an empty string),
then XmTextGetTopCharacter returns 0.
Suppose that the XmNtopCharacter resource has been
set to a value greater than the number of characters in
the text widget. In this case,
XmTextGetTopCharacter returns an
XmTextPosition value identifying
the position of the first character in the last line of text in a
multiline case; otherwise, it identifies the position of the last
character in the line.
For a complete definition of Text and its associated resources, see
Specifies the Text widget ID