Don't hide CI dropdown behind diff summary

Occurs on a repository commit page

app/assets/stylesheets/pages/pipelines.scss

@@ -978,7 +978,6 @@ button.mini-pipeline-graph-dropdown-toggle {
  * Top arrow in the dropdown in the mini pipeline graph
  */
 .mini-pipeline-graph-dropdown-menu {
-  z-index: 200;
 
   &::before,
   &::after {

changelogs/unreleased/ci-dropdown-hidden-bug.yml

+---
+title: Don't hide CI dropdown behind diff summary
+merge_request:
+author: gfyoung
+type: fixed