This just uses a narrow shadow for now.
691c96d Adwaita: Initial theming for scrollable indication
gtk/theme/Adwaita/_common.scss | 19 ++++++++++
gtk/theme/Adwaita/gtk-contained-dark.css | 61 ++++++++++++++++++------------
gtk/theme/Adwaita/gtk-contained.css | 61 ++++++++++++++++++------------
3 files changed, 93 insertions(+), 48 deletions(-)
Upstream: git.gnome.org