So that the tab height does not depend on the scrollbar height (which is subject to the desktop theme)

Change-Id: Ie2d787528152aed92c9943fe76955bf499d7d2a9
Reviewed-on: https://gerrit.libreoffice.org/21707Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>