Merge branch 'startup-css-flickering-of-content' into 'master'

Improve startup.css cloaking

See merge request gitlab-org/gitlab!39115

app/assets/stylesheets/application.scss

@@ -53,7 +53,5 @@
 }
 
 /* Rules for overriding cloaking in startup-general.scss */
-.container-limited,
-.modal-dialog {
-  display: block;
-}
+@import 'startup/cloaking';
+@include cloak-startup-scss(block);

app/assets/stylesheets/startup/_cloaking.scss

+/**
+  Prevent flashing of content when using startup.css
+ */
+@mixin cloak-startup-scss($display) {
+  // Breadcrumbs and alerts on the top of the page
+  .content-wrapper > .alert-wrapper,
+  // Content on pages
+  #content-body,
+  // Prevent flashing of haml generated modal contents
+  .modal-dialog {
+    display: $display;
+  }
+}