This had the same problem as the list node widget, fixed in commit
fb9c7e31 (tdf#97822 vcl opengl gtk2: fix
missing list node widget, 2019-04-12). Render output was missing in the
GL / SAL_GTK_USE_PIXMAPPAINT=1 case.

Change-Id: I24252c697523214a93f715759e242d47352c8341
Reviewed-on: https://gerrit.libreoffice.org/70760Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins