diff --git a/templates/zen-userChrome.css b/templates/zen-userChrome.css
index 30e988d..675b152 100644
--- a/templates/zen-userChrome.css
+++ b/templates/zen-userChrome.css
@@ -54,8 +54,10 @@
   color: {{colors.on_surface.default.hex}} !important;
 }
 
-/* Popup panels (menus, extensions etc) */
-.panel-arrowcontent, panel, #appMenu-popup {
+/* Popup panels (menus, extensions etc). Not the middle-click autoscroll
+   popup: it is a panel too, but only its round content part is drawn — a
+   background on the panel itself shows up as a square behind the circle. */
+.panel-arrowcontent, panel:not(.autoscroller), #appMenu-popup {
   background-color: {{colors.surface_container.default.hex}} !important;
   color: {{colors.on_surface.default.hex}} !important;
 }