if the scrollbar sizes haven't been calculated yet then the optimalsize will be
wrong, so it's necessary to enforce calculating them in that case.

(cherry picked from commit f41a2804)

Change-Id: I12e95c3594ffbb508c7f79bd512e07aaf44a6eab
Reviewed-on: https://gerrit.libreoffice.org/1630Reviewed-by: Kohei Yoshida <kohei.yoshida@gmail.com>
Tested-by: Kohei Yoshida <kohei.yoshida@gmail.com>