[gtk/circular-menubutton] Allow circular menubuttons

[Matthias Clasen <matthiasc src gnome org>]

commit 1cebc4316ba0554a10ee1c10e0ef868ba7076cea
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Jan 26 00:17:08 2021 -0500

    Allow circular menubuttons
    
    Fixes #3523

 gtk/gtkmenubutton.c            | 3 +++
 gtk/theme/Adwaita/_common.scss | 1 +
 2 files changed, 4 insertions(+)
---
diff --git a/gtk/gtkmenubutton.c b/gtk/gtkmenubutton.c
index 255a11c658..7204544830 100644
--- a/gtk/gtkmenubutton.c
+++ b/gtk/gtkmenubutton.c
@@ -120,6 +120,9 @@
  * the menu will appear in. The CSS is expected to provide a suitable
  * image for each of these cases using the -gtk-icon-source property.
  *
+ * Optionally, the menubutton node can carry the .circular style class
+ * to request a round appearance.
+ *
  * # Accessibility
  *
  * GtkMenuButton uses the #GTK_ACCESSIBLE_ROLE_BUTTON role.
diff --git a/gtk/theme/Adwaita/_common.scss b/gtk/theme/Adwaita/_common.scss
index 05877b1dc7..77790d7528 100644
--- a/gtk/theme/Adwaita/_common.scss
+++ b/gtk/theme/Adwaita/_common.scss
@@ -746,6 +746,7 @@ button {
 
   .linked.vertical > & { @extend %linked_vertical; }
 
+  menubutton.circular &,
   &.circular { // force circular button shape
     min-width: 32px;
     min-height: 32px;