Make perfbar z-index above modal background

app/assets/stylesheets/framework/variables_overrides.scss

@@ -18,3 +18,4 @@ $success: $green-500;
 $info: $blue-500;
 $warning: $orange-500;
 $danger: $red-500;
+$zindex-modal-backdrop: 1040;

app/assets/stylesheets/performance_bar.scss

 @import 'framework/variables';
+@import 'framework/variables_overrides';
 @import 'peek/views/rblineprof';
 
 #js-peek {
@@ -6,7 +7,7 @@
   left: 0;
   top: 0;
   width: 100%;
-  z-index: 1039;
+  z-index: #{$zindex-modal-backdrop + 1};
 
   height: $performance-bar-height;
   background: $black;

changelogs/unreleased/50552-unable-to-close-performance-bar.yml

+---
+title: Fix performance bar modal position
+merge_request: 21577
+author:
+type: fixed