num-scroll-lines in editor<%>
Reports the number of scroll positions available within the editor.
For text% objects:
Calling this method may force the recalculation of location
information, even if the editor currently has delayed refreshing (see
refresh-delayed?). If the editor is not displayed and the editor has a
maximum width, line breaks are calculated as for
line-start-position (which handles specially
the case of no display when the editor has a maximum width).