while the behavior was in place, the specific theming for overlay scrollbars was missing, and content was hidden.
5cce205 HC: implement overlay scrollbar theming
gtk/theme/HighContrast/_common.scss | 37 +++++++++++++++++++++++++++++++++++
gtk/theme/HighContrast/gtk.css | 19 ++++++++++++++++++
2 files changed, 56 insertions(+)
Upstream: git.gnome.org