Tweak some scrollbox styling to fix fallout from bug 1913322

chrome/multi-row_tabs.css

@@ -26,6 +26,7 @@ See the above repository for updates as well as full license text. */
   #scrollbutton-up,
   #scrollbutton-down{ display: var(--scrollbutton-display-model,initial) }
 
+  scrollbox[part][orient="horizontal"] > slot,
   scrollbox[part][orient="horizontal"]{
     display: flex;
     flex-wrap: wrap;
@@ -36,6 +37,9 @@ See the above repository for updates as well as full license text. */
     scrollbar-gutter: stable;
     scroll-snap-type: y mandatory;
   }
+  scrollbox[part][orient="horizontal"] > slot{
+    overflow-x: hidden;
+  }
 }
 
 .scrollbox-clip[orient="horizontal"],