Fill the terminal box from right to left to avoid

Gtk-WARNING **: Allocating size to GtkScrollbar 0x55f6d54b0200 without
    calling gtk_widget_get_preferred_width/height(). How does the code
    know the size to allocate?

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Message-id: 902aaef8-d20e-0530-dea2-cdfe3db33ff3@web.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>