Passing a right edge co-ordinate to a width param, ever since
   commit 4849fb6d
   Date:   Mon May 10 14:47:20 2004 +0000
   INTEGRATION: CWS nwf (1.25.36); FILE MERGED

Probably only works because the button is on the right edge, and
consequently the width will be clamped to the widget area.

Change-Id: I28a88cd298bc5beb20162f457c7cc125e1d0303a
Reviewed-on: https://gerrit.libreoffice.org/72231
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>