The reason for this is that the height of the scrollbar is theme-dependent
and makes the tabs unreadable if it's too small.

The same has already been done for Calc (fdo#36772).

Change-Id: Ibe22cf5e7d5863ccf276c19bfa969f5b5fc16a5a
Reviewed-on: https://gerrit.libreoffice.org/11480Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>