diff --git a/templates/zen-userChrome.css b/templates/zen-userChrome.css
index 675b152..e556523 100644
--- a/templates/zen-userChrome.css
+++ b/templates/zen-userChrome.css
@@ -62,6 +62,14 @@
   color: {{colors.on_surface.default.hex}} !important;
 }
 
+/* Said outright as well, so it also beats the old `panel` rule in a
+   userChrome.css parsed before this change. Gecko picks the popup's widget
+   transparency from this background when the popup is first shown and the
+   autoscroller is reused for the session, so a stale one needs a Zen restart. */
+#autoscroller {
+  background: transparent !important;
+}
+
 /* Content area containers */
 #browser, #appcontent, #tabbrowser-tabbox,
 #tabbrowser-tabpanels, #zen-appcontent-wrapper,