Set z-index for sticky diff box lower (#12537)

Fix #12525

Signed-off-by: Andrew Thornton <art27@cantab.net>

web_src/less/_repository.less

@@ -1579,7 +1579,7 @@
         &.sticky {
             position: sticky;
             top: 0;
-            z-index: 800;
+            z-index: 8;
             margin-bottom: 10px;
             border-bottom: 1px solid #d4d4d5;
             padding-left: 5px;