Merge branch 'slashmanov/fix-diff-files-sticky-header-background' into 'master'

Fix diff files sticky header transparent background

See merge request gitlab-org/gitlab!80749

app/assets/stylesheets/framework/diffs.scss

@@ -859,6 +859,8 @@ table.code {
 }
 
 .diff-files-changed {
+  background-color: $body-bg;
+
   .inline-parallel-buttons {
     @include gl-relative;
     z-index: 1;