In order to reproduce:
 Start LO and open a new text document.
 If necessary decrease the zoom factor in order to display the whole width of the document (= horizontal scroll bar is not displayed).
 Uncheck Option "LibreOffice Writer -> View -> Horizontal Scrollbar". Then "OK".
 Check Option "LibreOffice Writer -> View -> Horizontal Scrollbar". Then "OK". An empty grey bar is displayed at the bottom of the working area !!! Expected behaviour: No empty bar, area should be used as working area.
Platform (if different from the browser):
Browser: Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:184.108.40.206) Gecko/20111212 Firefox/3.6.25
Thank you for the detailed report!
Fixed in both master and libreoffice-3-5 (will be in LibO 3.5.1).
Checked with Version 3.6.3. Function is OK. From my point of view the bug can be closed.