Scrollbar: Show scrollbar on only on hover (#22386)

packages/grafana-ui/src/components/CustomScrollbar/_CustomScrollbar.scss

@@ -31,11 +31,21 @@
   .thumb-vertical {
   .thumb-vertical {
     @include gradient-vertical($scrollbarBackground, $scrollbarBackground2);
     @include gradient-vertical($scrollbarBackground, $scrollbarBackground2);
     border-radius: 6px;
     border-radius: 6px;
+    opacity: 0;
   }
   }
 
 
   .thumb-horizontal {
   .thumb-horizontal {
     @include gradient-horizontal($scrollbarBackground, $scrollbarBackground2);
     @include gradient-horizontal($scrollbarBackground, $scrollbarBackground2);
     border-radius: 6px;
     border-radius: 6px;
+    opacity: 0;
+  }
+
+  &:hover {
+    .thumb-vertical,
+    .thumb-horizontal {
+      opacity: 0.8;
+      transition: opacity 0.3s ease-in-out;
+    }
   }
   }
 
 
   // page scrollbar should stick to left side to aid hitting it
   // page scrollbar should stick to left side to aid hitting it