only subtract height of hscroll if visible

otherwise infinite loop in extensions update check

Change-Id: I0dbf4f34208f1bf0854e365006a79470b26571f3
(cherry picked from commit 749f7548b4ee746d69c20a828ea6d3b853eba8d3)
Reviewed-on: https://gerrit.libreoffice.org/11033
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>