the darker bottom border used on buttons looks bad on circular ones so now a gradient clipped on the border-box and a transparent border is used in that partcular case. See https://bugzilla.gnome.org/show_bug.cgi?id=771205 for details.
24cb9d4 Adwaita: make circular button border looking nice
gtk/theme/Adwaita/_common.scss | 22 ++++++++++++++++++++++
gtk/theme/Adwaita/gtk-contained-dark.css | 26 +++++++++++++++++++++++++-
gtk/theme/Adwaita/gtk-contained.css | 26 +++++++++++++++++++++++++-
3 files changed, 72 insertions(+), 2 deletions(-)
Upstream: git.gnome.org